Chapter 4

FOOTNOTES:[7]The following lines are from a letter of mine, which appeared in theTimesof April 13, some time after the present article was written:—'A few months ago I said in these columns that the determination of the sun's distance, then recently communicated to Parliament—namely, 93,375,000 miles—was probably some 800,000 miles too great; and I spoke of the method on which the determination was based as to some degree discredited by the wide range of difference both between that result and the mean of the best former measurements, and between the several results of which that one was itself the mean. Captain Tupman, as straightforward as he is skilful and zealous, announces as the result of a re-examination of the British observations a distance about 600,000 miles less than the above, or, more exactly, about 92,790,000 miles, as the sun's mean distance. But while he obtains from the ingress observations a mean distance of only 92,300,000 miles, he obtains from the egress observations a mean distance of about 93,040,000 miles; and the value, 92,790,000 miles, is only obtained as the mean of these two values duly weighted, the egress observations being more satisfactory than the ingress observations. 'It appears to me that the doubts which I formerly expressed as to the trustworthiness of the method employed, are to some degree justified.'To the general public it will be more interesting to inquire what probably is the true mean distance of the sun. To this it may be replied that in all probability the sun's mean distance does not lie so much as 600,000 miles on either side of the value 92,300,000 miles' (it should be 92,400,000).

FOOTNOTES:

[7]The following lines are from a letter of mine, which appeared in theTimesof April 13, some time after the present article was written:—'A few months ago I said in these columns that the determination of the sun's distance, then recently communicated to Parliament—namely, 93,375,000 miles—was probably some 800,000 miles too great; and I spoke of the method on which the determination was based as to some degree discredited by the wide range of difference both between that result and the mean of the best former measurements, and between the several results of which that one was itself the mean. Captain Tupman, as straightforward as he is skilful and zealous, announces as the result of a re-examination of the British observations a distance about 600,000 miles less than the above, or, more exactly, about 92,790,000 miles, as the sun's mean distance. But while he obtains from the ingress observations a mean distance of only 92,300,000 miles, he obtains from the egress observations a mean distance of about 93,040,000 miles; and the value, 92,790,000 miles, is only obtained as the mean of these two values duly weighted, the egress observations being more satisfactory than the ingress observations. 'It appears to me that the doubts which I formerly expressed as to the trustworthiness of the method employed, are to some degree justified.'To the general public it will be more interesting to inquire what probably is the true mean distance of the sun. To this it may be replied that in all probability the sun's mean distance does not lie so much as 600,000 miles on either side of the value 92,300,000 miles' (it should be 92,400,000).

[7]The following lines are from a letter of mine, which appeared in theTimesof April 13, some time after the present article was written:—

'A few months ago I said in these columns that the determination of the sun's distance, then recently communicated to Parliament—namely, 93,375,000 miles—was probably some 800,000 miles too great; and I spoke of the method on which the determination was based as to some degree discredited by the wide range of difference both between that result and the mean of the best former measurements, and between the several results of which that one was itself the mean. Captain Tupman, as straightforward as he is skilful and zealous, announces as the result of a re-examination of the British observations a distance about 600,000 miles less than the above, or, more exactly, about 92,790,000 miles, as the sun's mean distance. But while he obtains from the ingress observations a mean distance of only 92,300,000 miles, he obtains from the egress observations a mean distance of about 93,040,000 miles; and the value, 92,790,000 miles, is only obtained as the mean of these two values duly weighted, the egress observations being more satisfactory than the ingress observations. 'It appears to me that the doubts which I formerly expressed as to the trustworthiness of the method employed, are to some degree justified.

'To the general public it will be more interesting to inquire what probably is the true mean distance of the sun. To this it may be replied that in all probability the sun's mean distance does not lie so much as 600,000 miles on either side of the value 92,300,000 miles' (it should be 92,400,000).

THE PAST HISTORY OF OUR MOON.

The moon, commonly regarded as a mere satellite of the earth, is in truth a planet, the least member of that family of five bodies circling within the asteroidal zone, to which astronomers have given the name of the terrestrial planets. There can be no question that this is the true position of the moon in the solar system. In fact, the fashion of regarding her as a mere attendant of our earth may be looked upon as the last relic of the old astronomy in which our earth figured as the fixed centre of the universe, and the body for whose sake all the celestial orbs were fashioned. In this aspect, also, the moon is a far more interesting object of research than when viewed as belonging to another and an inferior order. We are able to recognise, in her, appearances probably resulting from the relative smallness of her dimensions, and hence to derive probable information as to the condition of other orbs in the solar system which fall below the earth in point of size. Precisely as the study of the giant planets, Jupiter and Saturn, has led astronomers to infer that certain peculiarities must result from vastness of dimensions, so the study of the dwarf planets, Mars, our moon, and Mercury, may indicate the relations we are to associate with inferiority of size.

This thought immediately introduces us to another conception, which causes us to regard with even greater interest the evidence afforded by the moon's present condition. It can scarcely be questioned that the size of any member of the solar system, or rather the quantity of matter in itsorb, assigns, so to speak, the duration of that orb's existence, or rather of the various stages of that existence. The smaller body must cool more rapidly than the larger, and hence the various periods during which the former is fit for this or that purpose of planetary life (I speak with purposed vagueness here) are shorter than the corresponding periods in the life of the latter. Thus the sun, viewed in this way, is the youngest member of the solar system, while the tiniest members of the asteroid family, if not the oldest in reality, are the oldest to which the telescope has introduced us. Jupiter and Saturn come next to the sun in youth; they are still passing through the earliest stages of planetary existence, even if we ought not rather to adopt that theory of their condition which regards them as subordinate suns, helping the central sun to support life on the satellites which circle around them. Uranus and Neptune are in a later stage, and perchance when telescopes have been constructed large enough to study these planets with advantage, we may learn something of that stage, interesting as being intermediate to the stages through which our earth and Venus on the one hand, and the giant brothers Jupiter and Saturn on the other, are at present passing. After our earth and Venus, which are probably at about the same stage of planetary development (though owing to the difference in their position they may not be equally adapted for the support of life), we come to Mars and Mercury, both of which must be regarded as in all probability much more advanced and in a sense more aged than the earth on which we live. In a similar sense,—even as an ephemeron is more aged after a few hours of existence than a man after as many years,—the small planet which we call 'our moon' may be described as in the very decrepitude of planetary existence, nay (some prefer to think), as even absolutely dead, though its lifeless body still continues to advance upon its accustomed orbit, and to obey the law of universal attraction.

Considerations such as these give singular interest to the discussion of the past history of our moon, though they addto the difficulty of interpreting the problems she presents to us. For we have manifestly to differentiate between the effects due to the moon's relative smallness on the one hand, and those due to her great age on the other. If we could believe the moon to be an orb which simply represents the condition to which our earth will one day attain, we could study her peculiarities of appearance with some hope of understanding how they had been brought about, as well as of learning from such study the future history of our own earth. But clearly the moon has had another history than our earth. Her relative smallness has led to relations such as the earth never has presented and never will present. If our earth is, as astronomers and physicists believe, to grow dead and cold, all life perishing from her surface, it is tolerably clear, from what we already know of her history, that the appearance she will present in her decrepitude will be utterly unlike that presented by the moon. Grant that after the lapse of enormous time-intervals the oceans now existing on the earth will be withdrawn beneath her solid crust, and even (which seems incredible) that at a more distant future the atmosphere now surrounding her will have become greatly reduced in quantity, either by similar withdrawal or in any other manner, yet the surface of the earth would present few features of resemblance to that of the moon. Viewed from the distance at which we view the moon, there would be few crateriform mountains indeed compared with those on the moon; those visible would be small by comparison with lunar craters even of medium dimensions; and the radiated regions seen on the moon's surface would have no discernible counterpart on the surface of the earth. The only features of resemblance, under the imagined conditions, would be probably the partially flat sea bottoms (though these would bear a different proportion to the more elevated regions) and the mountain ranges, the only terrestrial features of volcanic disturbance which would be relatively more important than their lunar counterparts.

I do not purpose, however, to discuss the probable futureof the earth, having only indicated the differences just touched upon in order to remind the reader at the outset that we have not in 'the moon' a representation of the earth at any stage of her history. Other and different relations are presented for our consideration, although it may well be that by carefully discussing them we may learn somewhat respecting our earth, as also respecting the past history and future development of the solar system.

It appears reasonable to regard the moon, after her first formation as a distinct orb, as presenting the same general characteristics that we ascribe to our earth in its primary stage as a planet. In one respect the moon, even at that early stage, may have differed from the earth. I refer to its rotation, the correspondence between which and its revolution may probably have existed from the moon's first formation. But this would not materially have affected the relations with which we have to deal at present. We may apply, then, to the moon the arguments which have been applied to the discussion of the first stages of our earth's history.

Adopting this view, we see that at the first stage of its existence as an independent planet, the moon must have been an intensely heated gaseous globe, glowing with inherent light, and undergoing a process of condensation, 'going on at first at the surface only, until by cooling it must have reached the point where the gaseous centre was exchanged for one of combined and liquefied matter.' To apply now to the moon at this stage the description which Dr. Sterry Hunt gives of the earth. 'Here commences the chemistry of the moon. So long as the gaseous condition of the moon lasted, we may suppose the whole mass to have been homogeneous; but when the temperature became so reduced that the existence of chemical compounds at the centre became possible, those which were most stable at the elevated temperature then prevailing would be first formed. Thus, for example, while compounds of oxygen with mercury, or even with hydrogen, could notexist, oxides of silicon, aluminium, calcium, magnesium, and iron, might be formed and condensed in a liquid form at the centre of the globe. By progressive cooling still other elements would be removed from the gaseous mass, which would form the atmosphere of the non-gaseous nucleus.' 'The processes of condensation and cooling having gone on until those elements which are not volatile in the heat of our ordinary furnaces were condensed into a liquid form, we may here inquire what would be the result on the mass of a further reduction of temperature. It is generally assumed that in the cooling of a liquid globe of mineral matter congelation would commence at the surface, as in the case of water; but water offers an exception to most other liquids, inasmuch as it is denser in the liquid than in the solid form. Hence, ice floats on water, and freezing water becomes covered with a layer of ice which protects the liquid below. Some metals and alloys resemble water in this respect. With regard to most other earthy substances, and notably the various minerals and earthy compounds like those which may be supposed to have made up the mass of the molten globe, the case is entirely different. The numerous and detailed experiments of Charles Deville and those of Delesse, besides the earlier ones of Bischoff, unite in showing that the density of fused rocks is much less than that of the crystalline products resulting from their slow cooling, these being, according to Deville, from one-seventh to one-sixteenth heavier than the fused mass, so that if formed at the surface they would, in obedience to the laws of gravity, tend to sink as soon as formed.'

Here it has to be noted that possibly there existed a period (for our earth as well as for the moon) during which, notwithstanding the relations indicated by Dr. Hunt, the exterior portions of the moon were solid, while the interior remained liquid. A state of things corresponding to what we recognise as possible in the sun may have existed. For although undoubtedly any liquid matter forming in the sunsinks in obedience to the laws of gravity towards the centre, yet the greater heat which it encounters as it sinks must vapourise it, notwithstanding increasing pressure, so that it can only remain liquid near the region where rapid radiation allows of sufficient cooling to produce liquefaction. And in the same way we may conceive that the solidification taking place at any portion of the surface of the moon's or the earth's liquid globe, owing to rapid radiation of heat thence, although it might be followed immediately by the sinking of the solidified matter, would yet result in the continuance (rather than the existence) of a partially solid crust. For the sinking solid matter, though subjected to an increase of pressure (which, in the case of matter expanding on liquefaction, would favour solidification), would nevertheless, owing to the great increase of heat, become liquefied, and, expanding, would no longer be so much denser[8]than the liquid through which it was sinking as to continue to sink rapidly.

Nevertheless, it is clear that after a time the heat of the interior parts of the liquid mass would no longer suffice to liquefy the solid matter descending from the surface, and then would commence the process of aggregation at the centre described by Dr. Hunt. The matter forming the solid centre of the earth consists probably of metallic and metalloidal compounds of elements denser than those forming the known portions of the earth's crust.[9]In the case of the moon, whose mean density is very little greater than the mean density of the matter forming the earth's crust, wemust assume that the matter forming the solid nucleus at that early stage was relatively less in amount, or else that we may attribute part of the difference to the comparatively small force with which lunar gravity operated during various stages of contraction and solidification.

In the case of the moon, as in that of the earth, before the last portions became solidified, there would exist a condition of imperfect liquidity, as conceived by Hopkins, 'preventing the sinking of the cooled and heavier particles, and giving rise to a superficial crust, from which solidification would proceed downwards. There would thus be enclosed between the inner and outer solid parts a portion of uncongealed matter,' which may be supposed to have retained its liquid condition to a late period, and to have been the principal seat of volcanic action, whether existing in isolated reservoirs or subterranean lakes, or whether, as suggested by Scrope, forming a continuous sheet surrounding the solid nucleus.

Thus far we have had to deal with relations more or less involved in doubt. We have few means of forming a satisfactory opinion as to the order of the various changes to which, in the first stages of her existence as a planet, our moon was subject. Nor can we clearly define the nature of those changes. In these matters, as with the corresponding processes in our earth's case, there is much room for variety of opinion.

But few can doubt that, by whatever processes such condition may have been attained, the moon, when her surface began to form itself into its present appearance, consisted of a globe partially molten surrounded by a crust at least partially solidified. Some portions of the actual surface may have remained liquid or viscous later than others but at length the time must have arrived when the radiating surface was almost wholly solid. It is from this stage that we have to trace the changes which have led to the present condition of the moon's surface.

It can scarcely be questioned that those seismologists arein the right who have maintained in recent times the theory that in the case of a cooling globe, such as the earth or moon at the stage just described, the crust would in the first place contract more quickly than the nucleus, while later the nucleus would contract more quickly than the crust. This amounts, in fact, to little more than the assertion that the process of heat radiation from the surface would be more rapid, and so last a shorter time than the process of conduction by which in the main the nucleus would part with its heat. The crust would part rapidly with its heat, contracting upon the nucleus; but the very rapidity (relative) of the process, by completing at an early stage the radiation of the greater portion of the heat originally belonging to the crust, would cause the subsequent radiation to be comparatively slow, while the conduction of heat from the nucleus to the crust would take place more rapidly, not only relatively but actually.

Now it is clear that the results accruing during the two stages into which we thus divide the cooling of the lunar globe would be markedly different. During the first stage forces of tension (tangential) would be called to play in the lunar crust; during the later stage the forces would be those of pressure.

Taking the earlier stage, during which the forces would be tensional, let us consider in what way these forces would operate.

At the beginning, when the crust would be comparatively thin, I conceive that the more general result of the rapid contraction of the crust would be the division of the crust into segments, by the formation of numerous fissures due to the lateral contraction of the thin crust. The molten matter in these fissures would film over rapidly, however, and all the time the crust would be growing thicker and thicker, until at length the formation of distinct segments would no longer be possible. The thickening crust, plastic in its lower strata, would now resist more effectively the tangential tensions, and when yielding would yield in a different manner.It was at this stage, in all probability, that processes such as those illustrated by Nasmyth's globe experiments took place, and that from time to time the crust yielded at particular points, which became the centres of systems of radiating fissures. Before proceeding, however, to consider the results of such processes, let it be noted that we have seen reason to believe that among the very earliest lunar formations would be rifts breaking theancientsurface of the lunar crust. I distinguish in this way the ancient surface from portions of surface whereof I shall presently have to speak as formed at a later time.

Now let us conceive the somewhat thickened crust contracting upon the partially fluid nucleus. If the crust were tolerably uniform in strength and thickness we should expect to find it yielding (when forced to yield) at many points, distributed somewhat uniformly over its extent. But this would not be the case if—as we might for many reasons expect—the crust were wanting in uniformity. There would be regions where the crust would be more plastic, and so readier to yield to the tangential tensions. Towards such portions of the crust the liquid matter within would tend, because there alone would room exist for it. The down-drawing, or rather in-drawing, crust elsewhere would force away the liquid matter beneath, towards such regions of less resistance, which would thus remain at (and be partly forced to) a higher level. At length, however, the increasing tensions thus resulting would have their natural effect; the crust would break open at the middle of the raised region, and in radiating rifts, and the molten matter would find vent through the rifts as well as at the central opening. The matter so extruded, being liquid, would spread, so that—though the radiating nature of the rifts would still be indicated by the position of the extruded matter—there would be no abrupt changes of level. It is clear, also, that so soon as the outlet had been formed the long and slowly sloping sides of the region of elevation would gradually sink, pressing the liquid matter below towards the centre of outlet,whence it would continue to pour out so long as this process of contraction continued. All round the borders of the aperture the crust would be melted, and would continue plastic long after the matter which had filled the fissures and flowed out through them had solidified. Thus there would be formed a wide circular orifice, which would from the beginning be considerably above the mean level of the moon's surface, because of the manner in which the liquid matter within had been gathered there by the pressure of the surrounding slopes.[10]Moreover, around the orifice, the matteroutflowing as the crust continued to contract would form a raised wall. Until the time came when the liquid nucleus began to contract more rapidly than the crust, the large crateriform orifice would be full to the brim (or nearly so), at all times, with occasional overflows: and as a writer who has recently adopted this theory has remarked, 'We should ultimately have a large central lake of lava surrounded by a range of hills, terraced on the outside,—the lake filling up the space they enclosed.'

The crust might burst in the manner here considered, at several places at the same—or nearly the same—time, the range of the radiating fissures, depending on the extent of the underlying lakes of molten matter thus finding their outlet; or there might be a series of outbursts at widely separated intervals of time and at different regions, gradually diminishing in extent as the crust gradually thickened and the molten matter beneath gradually became reduced in relative amount. Probably the latter view should be accepted, since, if we consider the three systems of radiations from Copernicus, Aristarchus, and Kepler, which were manifestly not formed contemporaneously, but in the order in which their central craters have just been named, we see that their dimensions diminished as their date of formation was later. According to this view we should regard the radiating system from Tycho as the oldest of all these formations.

At this very early stage of the moon's history, then, we regard the moon as a somewhat deformed spheroid, the regions whence the radiations extended being the highest parts, and the regions farthest removed from the ray centres being the lowest.[11]To these lower regions whatever wasliquid on the moon's surface would find its way. The down-flowing lava would not be included in this description, as being rather viscous than liquid; but if any water existed at that time it would occupy the depressed regions which at the present time are called Maria or Seas.

It is a question of some interest, and one on which different opinions have been entertained, whether the moon at any stage of its existence had oceans and an atmosphere corresponding in relative extent to those of the earth. It appears to me that, apart from all the other considerations which have been suggested in support of the view that the moon formerly had oceans and an atmosphere, it is exceedingly difficult to imagine how, under any circumstances, a globe so large as the moon could have been formed under conditions not altogether unlike, as we suppose, those under which the earth was formed (having a similar origin, and presumably constructed of the same elements), without having oceans and an atmosphere of considerable extent. The atmosphere would not consist of oxygen and nitrogen only or chiefly, any more than, in all probability, the primeval atmosphere of our own earth was so constituted. We may adopt some such view of the moon's atmosphere—mutatis mutandis—as Dr. Sterry Hunt has adopted respecting the ancient atmosphere of the earth. Hunt, it will be remembered, bases his opinion on the former condition of the earth by conceiving an intense heat applied to the earth as now existing, and inferring the chemical results. 'To the chemist,' he remarks, 'it is evident that from such a process applied to our globe would result the oxidation of all carbonaceous matter; the conversion of all carbonates, chlorides, and sulphates into silicates; and the separation of the carbon, chlorine, andsulphur in the form of acid gases; which, with nitrogen, watery vapour, and an excess of oxygen, would form an exceedingly dense atmosphere. The resulting fused mass would contain all the bases as silicates, and would probably nearly resemble in composition certain furnace-slags or basic volcanic glasses. Such we may conceive to have been the nature of the primitive igneous rock, and such the composition of the primeval atmosphere,which must have been one of very great density.' All this, with the single exception of the italicised remark, may be applied to the case of the moon. The lunar atmosphere would not probably be dense at that primeval time, even though constituted like the terrestrial atmosphere just described. It would perhaps have been as dense, or nearly so, as our present atmosphere. Accordingly condensation would take place at a temperature not far from the present boiling-point, and the lower levels of the half-cooled crust would be drenched with a heated solution of hydrochloric acid, whose decomposing action would be rapid, though not aided—as in the case of our primeval earth—by an excessively high temperature. 'The formation of the chlorides of the various bases and the separation of silica would go on until the affinities of the acid were satisfied.' 'At a later period the gradual combination of oxygen with sulphurous acid would eliminate this from the atmosphere in the form of sulphuric acid.' 'Carbonic acid would still be a large constituent of the atmosphere, but thenceforward (that is, after the separation of the compounds of sulphur and chlorine from the air) there would follow the conversion of the complex aluminous silicates, under the influence of carbonic acid and moisture, into a hydrated silicate of alumina or clay, while the separated lime, magnesia, and alkalies would be changed into bicarbonates, and conveyed to the sea in a state of solution.'

It seems to me that it is necessary to adopt some such theory as to the former existence of lunar oceans in order to explain some of the appearances presented by the so-calledlunar seas. As regards the present absence of water we may adopt the theory of Frankland, that the lunar oceans have withdrawn beneath the crust as room was provided for them by the contraction of the nucleus. I think, indeed, that there are good grounds for looking with favour on the theory of Stanislas Meunier, according to which the oceans surrounding any planet—our own earth or Mars, for example—are gradually withdrawn from the surface to the interior. And in view of the enormous length of the time-intervals required for such a process, we must consider that while the process was going on the lunar atmosphere would not only part completely with the compounds of sulphur, chlorine, and carbon, but would be even still further reduced by chemical processes acting with exceeding slowness, yet effectively in periods so enormous. But without insisting on this consideration, it is manifest that—with very reasonable assumptions as to the density of the lunar atmosphere in its original complex condition—what would remain after the removal of the chief portion by chemical processes, and after the withdrawal of another considerable portion along with the seas beneath the lunar crust, would be so inconsiderable in quantity as to accord satisfactorily with the evidence which demonstrates the exceeding tenuity of any lunar atmosphere at present existing.

These considerations introduce us to the second part of the moon's history,—that corresponding to the period when the nucleus was contracting more rapidly than the crust.

One of the first and most obvious effects of this more rapid nuclear contraction would be the lowering of the level of the molten matter, which up to this period had been kept up to, or nearly up to, the lips of the great ringed craters. If the subsidence took place intermittently there would result a terracing of the interior of the ringed elevation, such as we see in many lunar craters. Nor would there be any uniformity of level in the several crater floors thus formed, since the fluid lava would not form parts of a single fluid mass (in which case, of course, the level of the fluidsurface would be everywhere the same), but would belong to independent fluid masses. Indeed it may be noticed that the very nature of the case requires us to adopt this view, since no other will account for the variety of level observed in the different lunar crater-floors. If these ceased to be liquid at different times, the independence of the fluid masses is by that very fact established; and if they ceased to be liquid at the same time, they must have been independent, since, if communication had existed between them, they would have shown the uniformity of surface which the laws of hydrostatics require.[12]

The next effect which would follow from the gradual retreat of the nucleus from the crust (setting aside the withdrawal of lunar seas) would be the formation of corrugations,—in other words, of mountain-ranges. Mallet describes the formation of mountain-chains as belonging to the period when 'the continually increasing thickness of the crust remained such that it was still as a whole flexible enough, or opposed sufficient resistance of crushing to admit of the uprise of mountain-chains by resolved tangential pressures.' Applying this to the case of the moon, I think it is clear that—with her much smaller orb and comparatively rapid rate of cooling—the era of the formation of mountain-chains would be a short one, and that these would therefore form a less important characteristic of her surface than of the earth's. On the other hand, the period of volcanic activity which would follow that of chain-formation would berelativelylong continued; for regarding this period as beginning when the thickness of the moon's crust had become too great to admit of adjustment by corrugation, the comparatively small pressure to which the whole mass of the moon had been subjected by lunar gravity, while it would on the one hand cause the period to have an earlier commencement (relatively), would on the other leave greater play to theeffects of contraction. Thus we can understand why the signs of volcanic action, as distinguished from the action to which mountain-ranges are due, should be far more numerous and important on the moon than on the earth.

I do not, however, in this place enter specially into the consideration of the moon's stage of volcanic activity, because already, in the pages of my Treatise on the Moon (Chapter VI.), I have given a full account of that portion of my present subject. I may make a few remarks, however, on the theory respecting lunar craters touched on in my work on 'The Moon.' I have mentioned the possibility that some among the enormous number of ring-shaped depressions which are seen on the moon's surface may have been the result of meteoric downfalls in long past ages of the moon's history. One or two critics have spoken of this view as though it were too fantastic for serious consideration. Now, though I threw out the opinion merely as a suggestion, distinctly stating that I should not care to maintain it as a theory, and although my own opinion is unfavourable to the supposition that any of the more considerable lunar markings can be explained in the suggested way, yet it is necessary to notice that on the general question whether the moon's surface has been marked or not by meteoric downfalls scarcely any reasonable doubts can be entertained. For, first, we can scarcely question that the moon's surface was for long ages plastic, and though we may not assign to this period nearly so great a length (350 millions of years) as Tyndall—following Bischoff—assigns to the period when our earth's surface was cooling from a temperature of 2000° C. to 200°, yet still it must have lasted millions of years; and, secondly, we cannot doubt that the process of meteoric downfall now going on is not a new thing, but, on the contrary, is rather the final stage of a process which once took place far more actively. Now Prof. Newton has estimated, by a fair estimate of observed facts, that each day on the average 400 millions of meteors fall, of all sizes down to the minutest discernible in a telescope, upon the earth'satmosphere, so that on the moon's unprotected globe—with its surface one thirteenth of the earth's—about 30 millions fall each day, even at the present time. Of large meteoric masses only a few hundreds fall each year on the earth, and perhaps about a hundred on the moon; but still, even at the present rate of downfall, millions of large massesmusthave fallen on the moon during the time when her surface was plastic, whileprobablya much larger number—including many much larger masses—must have fallen during that period. Thus, not only without straining probabilities, but by taking only the most probable assumptions as to the past, we have arrived at a result which compels us to believe that the moon's surface has been very much marked by meteoric downfall, while it renders it by no means unlikely that a large proportion of the markings so left would be discernible under telescopic scrutiny.

I would, in conclusion, invite those who have the requisite leisure to a careful study of the distribution of various orders of lunar marking. It would be well if the moon's surface were isographically charted, and the distribution of the seas, mountain ranges, and craters of different dimensions and character, of rills, radiating streaks, bright and dark regions, and so on, carefully comparedinter se, with the object of determining whether the different parts of the moon's surface were probably brought to their present condition during earlier or later periods, and of interpreting also the significance of the moon's characteristic peculiarities. In this department of astronomy, as in some others, the effectiveness of well-devised processes of charting has been hitherto overlooked.

FOOTNOTES:[8]It would still be somewhat denser, because under the circumstances it would be somewhat cooler.[9]It is thus, and not by the effects due to increasing pressure (effects which probably do not increase beyond a certain point), that we are to explain the fact that the earth's density as a whole is about twice the mean density of the matters which form its solid surface. It may be that this consideration, supported by the results of recent experimental researches, may give a significance hitherto not noted to the relatively small mean density of the moon.[10]I have occasion to make some remarks at this stage to avoid possible and (my experience has shown me) not altogether improbable misconception, or even misrepresentation. The theory enunciated above will be regarded by some, who may have read a certain review of my Treatise on the Moon, as totally different from what I have advocated in that work, and, furthermore, as a theory which I have borrowed from the aforesaid review. I should not be particularly concerned if I had occasion to modify views I had formerly expressed, since I apprehend that every active student of science should hope, rather than dread, that as his work proceeds he would form new opinions. But I must point out that earlier in my book I had advocated the theory urged above. After describing the radiations from Tycho and other craters, I proceed as follows in chapter iv.—'It appears to me impossible to refer these phenomena to any general cause but the reaction of the moon's interior overcoming the tension of the crust, and to this degree Nasmyth's theory seems correct; but it appears manifest, also, that the crust cannot have been fractured in the ordinary sense of the word. Since, however, it results from Mallet's investigations that the tension of the crust is called into play in the earlier stages of contraction, and its power to resist contraction in the later stages,—in other words, since the crust at first contracts faster than the nucleus, and afterwards not so fast as the nucleus,—we may assume that the radiating systems were formed in so early an era that the crust was plastic. And it seems reasonable to conclude that the outflowing matter would retain its liquid condition long enough (the crust itself being intensely hot) to spread widely,—a circumstance which would account at once for the breadth of many of the rays, and for the restoration of level to such a degree that no shadows are thrown. It appears probable, also, that not only (which is manifest) were the craters formed later which are seen around and upon the radiations, but that the central crater itself acquired its actual form long after the epoch when the rays were formed.'[11]Where several ray centres are near together, a region directly between two ray centres would be at a level intermediate between that of the ray centres and that of a region centrally placed within a triangle or quadrangle of ray centres; but the latter region might be at a higher level than another very far removed from the part where the ray centres were near together. For instance, the space in the middle of the triangle having Copernicus, Aristarchus, and Kepler at its angles (or more exactly between Milichius and Bessarion) is lower than the surface around Hortensius (between Copernicus and Kepler), but not so low as the Mare Imbrium, far away from the region of ray centres of which Copernicus, Aristarchus, and Kepler are the principal.[12]It is important to notice that we may derive from these considerations an argument as to the condition of the fluid matter now existing beneath the solid crust of the earth.

FOOTNOTES:

[8]It would still be somewhat denser, because under the circumstances it would be somewhat cooler.

[9]It is thus, and not by the effects due to increasing pressure (effects which probably do not increase beyond a certain point), that we are to explain the fact that the earth's density as a whole is about twice the mean density of the matters which form its solid surface. It may be that this consideration, supported by the results of recent experimental researches, may give a significance hitherto not noted to the relatively small mean density of the moon.

[10]I have occasion to make some remarks at this stage to avoid possible and (my experience has shown me) not altogether improbable misconception, or even misrepresentation. The theory enunciated above will be regarded by some, who may have read a certain review of my Treatise on the Moon, as totally different from what I have advocated in that work, and, furthermore, as a theory which I have borrowed from the aforesaid review. I should not be particularly concerned if I had occasion to modify views I had formerly expressed, since I apprehend that every active student of science should hope, rather than dread, that as his work proceeds he would form new opinions. But I must point out that earlier in my book I had advocated the theory urged above. After describing the radiations from Tycho and other craters, I proceed as follows in chapter iv.—'It appears to me impossible to refer these phenomena to any general cause but the reaction of the moon's interior overcoming the tension of the crust, and to this degree Nasmyth's theory seems correct; but it appears manifest, also, that the crust cannot have been fractured in the ordinary sense of the word. Since, however, it results from Mallet's investigations that the tension of the crust is called into play in the earlier stages of contraction, and its power to resist contraction in the later stages,—in other words, since the crust at first contracts faster than the nucleus, and afterwards not so fast as the nucleus,—we may assume that the radiating systems were formed in so early an era that the crust was plastic. And it seems reasonable to conclude that the outflowing matter would retain its liquid condition long enough (the crust itself being intensely hot) to spread widely,—a circumstance which would account at once for the breadth of many of the rays, and for the restoration of level to such a degree that no shadows are thrown. It appears probable, also, that not only (which is manifest) were the craters formed later which are seen around and upon the radiations, but that the central crater itself acquired its actual form long after the epoch when the rays were formed.'

[11]Where several ray centres are near together, a region directly between two ray centres would be at a level intermediate between that of the ray centres and that of a region centrally placed within a triangle or quadrangle of ray centres; but the latter region might be at a higher level than another very far removed from the part where the ray centres were near together. For instance, the space in the middle of the triangle having Copernicus, Aristarchus, and Kepler at its angles (or more exactly between Milichius and Bessarion) is lower than the surface around Hortensius (between Copernicus and Kepler), but not so low as the Mare Imbrium, far away from the region of ray centres of which Copernicus, Aristarchus, and Kepler are the principal.

[12]It is important to notice that we may derive from these considerations an argument as to the condition of the fluid matter now existing beneath the solid crust of the earth.

A NEW CRATER IN THE MOON.

Dr. Klein, a German astronomer, has recently called the attention of astronomers to a lunar crater some three miles wide, which had not before been observed, and which, he feels sure, was not in existence two years ago. Astronomers have long since given up all hope of tracing either the signs of actual life upon the moon or traces of the past existence of living creatures there. But there are still among them those who believe that by sedulous and careful scrutiny processes of material change may be recognised in that seemingly inert mass. In reality, perhaps, the wonder rather is that signs of change should not be often recognised, than that from time to time a new crater should appear or the walls of old craters fall in. The moon's surface is exposed to variations of temperature compared with which those affecting the surface of our earth are altogether trifling. It is true there is no summer or winter in the moon. Sir W. Herschel has spoken of the lunar seasons as though they resembled our own, but in reality they are very different. The sun's midday height at any lunar station is only about three degrees greater in summer than in winter; whereas our summer sun is 47° higher in the sky at noon than our winter sun. In fact, a midsummer's day on the moon does not differ more from a midwinter's day, as far as the sun's actual path on the sky is concerned, than with us the 17th of March differs from the 25th, or the 19th of September from the 27th. It is the change from day to night which chieflyaffects the moon's surface. In the lunar year of seasons, lasting 346-2/3 of our days, there are only 11¾ lunar days, each lasting 29¾ of ours. Thus day lasts more than a fortnight, and is followed by a night of equal length. Nor is this all. There is neither air nor moisture to produce such effects as are produced by our air and the moisture it contains in mitigating the heat of day and the cold of night. Under the sun's rays the moon's surface becomes hotter and hotter as the long lunar day proceeds, until at last its heat exceeds that of boiling water. But so soon as the sun has set the heat thus received is rapidly radiated away into space (no screen of moisture-laden air checking its escape), and long before lunar midnight a cold exists compared with which the bitterest weather ever experienced by Arctic voyagers would be oppressively hot. These are not merely theoretical conclusions, though even as such they could be thoroughly relied upon. The moon's heat has been measured by the present Lord Rosse (using his father's splendid six-feet mirror). He separated the heat which the moon simply reflects to us from that which her heated surface itself gives out (or, technically, he separated the reflected from the radiated heat), by using a glass screen which allowed the former heat to pass while it intercepted the latter. He thus found that about six-sevenths of the heat we receive from the moon is due to the heating of her own substance. From the entire series of observations it appeared that the change of temperature during the entire lunar day—that is, from near midnight to near midday on the moon—amounts to fully 500° Fahrenheit. If we assume that the cold at lunar midnight corresponds with about 250° below zero (the greatest cold experienced in Arctic travelling has never exceeded 140° below zero), it would follow that the midday heat is considerably greater than that of boiling water on the earth at the sea-level. But the range of change is not a matter of speculation. It certainly amounts to about 500°, and in whatever way we distribute it, we must admit, first, that no such life as we are familiar with could possibly exist on themoon; and, secondly, that the moon's crust must possess a life of its own, so to speak, expanding and contracting unceasingly and energetically. Professor Newcomb, by the way, in his fine work on Popular Astronomy, rejects the idea that the expansions and contractions due to these great changes of temperature can cause any disintegration at the present time. There might, he says, be bodies so friable that they would crumble, 'but whatever crumbling might thus be caused would soon be done with, and then no further change would occur.' For my own part, I cannot consider that such a surface as the moon at present possesses can undergo these continual expansions and contractions without slow disintegration. It seems to me also extremely probable that from time to time the overthrow of great masses, the breaking up of arched crater-floors, and other sudden changes discernible from the earth, might be expected to occur. Professor Newcomb has, I conceive, omitted to consider the enormous volumetric expansion as distinguished from mere lateral extension, resulting from the heating of the moon's crust to considerable depths. On a very moderate computation, the surface of the central region of the full moon must at that time rise above its mean position to such a degree that hundreds, if not thousands, of cubic miles of the moon's volume lie above the mean position of the surface there. At new moon—that is, at lunar midnight for the same region—the same enormous quantity of matter is correspondingly depressed. And though the actual range in vertical height at any given point may be small, we cannot doubt that the total effect produced by these constant oscillations is considerable. Years or centuries may pass without any great or sudden change, but from time to time such catastrophes must surely occur. I believe that all the cases of supposed change in the moon, if all were regarded as proved, could be thus fully accounted for without any occasion to assume the action of volcanic forces properly so called.

Before considering the evidence for the new lunarvolcano to which Dr. Klein has recently called the attention of astronomers, it may be well briefly to describe the condition of the moon's surface.

This surface, which is equal in extent to about that of the American Continent, or to Europe and Africa together (without their islands), is divided into two chief portions—the higher levels, which are in the main of lighter tint, and the lower levels, which are, almost without exception, dark. It may be remarked in passing that very erroneous ideas are commonly entertained respecting the moon's general colour. The moon is very far from being white, as many suppose. On the contrary, she is far more nearly black than white. It has been well remarked by Tyndall that if the moon were covered with black velvet (14,600,000 square miles of that material would be required for the purpose), she would still appear white to us, for we should see her disc projected on the blackness of star-strewn space. The actual tint of the moon as a whole is nearly the same as that of gray weathered sandstone. The brightest parts, however, are much whiter; Zöllner infers from his photometric experiments that they can be compared with the whitest of terrestrial substances. On the other hand, the darkest parts of the moon are probably far darker than porphyry, even if they are not so dark that on earth we should call them black. The fact that the low-lying parts of the moon are much darker than the higher regions is full of meaning, though hitherto its significance does not seem to have been much noticed. Either we must assume that these lower regions, the so-called seas (certainly now dry), are old sea bottoms, and owe their darkness to the quality of the matter deposited there in remote ages, or else we must suppose that the matter which last remained fluid when the moon's surface was consolidating was of darker material than the rest. For such matter would occupy the lowest lunar regions. There is here room for a very profitable study of the moon's aspect by geologists. I doubt not that, however different the general pasthistory of our earth may have been from the moon's, terrestrial regions exist where the characteristic features of the moon's surface are more or less closely illustrated. On the American continent, for example, there are peculiarities of geological formation which seem to correspond closely with some of the features of the lunar globe, presently to be noticed; and it seems to me not improbable that geologists might find in the study of certain regions in North America the means of interpreting the difference of tint between higher and lower levels on the moon. If so, light would probably be thrown on very difficult questions relating to the remote past, not only of the moon, but of our own earth.

The lunar feature which comes next in importance to the difference of tint between the so-called 'seas' and the higher lands is the existence of remarkable series of radiating streaks extending from certain important craters—centres probably of past disturbance. It is impossible to contemplate the disc of the full moon, as seen with a powerful telescope, without feeling that these systems of rays must have resulted from the operation of forces of the most stupendous nature, though as yet their true meaning is hid from us. They would be marvellous phenomena, even if they were not so mysterious—marvellous in their enormous extension, their singular brightness, and their manner of traversing 'seas,' craters, and mountain-ranges indifferently. But their chief marvel resides in the mysterious manner of their appearance as the moon approaches her full illumination. Other lunar features are most clearly recognised when the moon is not full, for then the shadows which afford our only means of estimating the height of lunar irregularities are clearly seen along the border between the bright and dark parts of her face, and we have only to wait until this border passes over any object we wish to study to obtain satisfactory evidence of its nature. It is quite otherwise with the rays. The regions occupied by these radiating streaks are neither raised nor depressedin such sort as either to throw shadows or to lie in shadow when surrounding regions are in sunlight. But when the moon approaches her full illumination, the radiating regions come into view, as bright streaks—bright even on the light-tinted lunar uplands. A mighty system of rays can be seen extending from the great crater Tycho in all directions. Other systems, scarcely less wonderful, extend from the battlemented crater, Copernicus, the brilliant Aristarchus, and the solitary Kepler. One ray from Tycho can be traced round nearly an entire hemisphere of the moon's surface. It is specially noteworthy of this great ray that, where it crosses the lunar Sea of Serenity, that great plain seems to be divided as by a sort of ridge line, the slope of the plain from either side of the ray's track being clearly discernible when the moon is near her first quarter.

What are these mysterious ray systems? How are we to explain the circumstance that though only the most tremendous forces seem competent to account for bands such as these, many miles broad and many hundreds of miles in length, there are yet none of the usual signs of the action of volcanic forces? If mighty rifts had been formed in the moon's crust by the outbursting action of a hot nucleus, or through the contraction of the crust on an unyielding nucleus (for the effect would be the same in either case), we should scarcely expect to find that after such rifts had formed no signs of any difference of level would appear. If lava flowed out all along the rift, one would imagine that it would form a long dyke which would throw an obvious shadow, except under full solar illumination. If the rift were not filled with lava, the bottom of the rift would certainly remain in shadow long after the surrounding region was illuminated. That lava should exactly fill the rift along its entire length seems incredible. This might happen by a strange chance in the case of a single rift, but not with all the rifts of a radiating system, still less with all the rifts of all the radiating systems. Yet I believe that neither astronomers nor geologists can form any other opinion respecting thesemysterious ray-systems than that they were caused by what Humboldt (speaking of the earth) calls the reaction of the interior on the crust. Nasmyth has admirably indicated their appearance, or rather their radiating form, by filling a globular glass shell with water, hermetically closed, and then freezing the water. The expansion of the water bursts the glass shell, and the lines of fracture are found to extend in a series of rays from the part of the shell which first gave way. But this experiment of itself does not explain the mystery of the lunar rays. Accepting the theory that the moon's crust yielded in some such way, we have still to explain how the rifts which were thus formed came to be covered over with matter lying nearly at the same level as the surrounding surface. It appears to me that the only available way of explaining this is somewhat as follows. First, from the way in which the streaks are covered like the surrounding region with craters, we may conclude that the streaks are older than any except the largest craters; from the great extension of many of them, we may safely infer that the lunar crust possessed a large measure of plasticity when they were formed (for otherwise it would have yielded over a smaller area). It was, therefore, probably still hot during the era (which may have lasted millions of years) to which the formation of the rifts belonged: accordingly the lava which flowed out through the rifts remained liquid for a considerable time, and was thus able to spread widely on either side of the rift, forming a broad band of lava-covered surface, instead of a steep and narrow dyke. This seems not only to account for the most striking peculiarity of the bands, but to accord well with all that is known about them, and even to suggest explanations of some other lunar features which had appeared perplexing. I understand that in certain regions of North America there are lava-covered rifts large enough to form geographical features, and, therefore, fairly comparable with the lunar radiating streaks. But as yet American geologists have not presented in an accessible form a description of the peculiar features of the Americancontinent; in fact, it may be doubted whether as yet the materials for such a description exist.

The mountain-ranges of the moon do not differ to any marked degree from those of our own earth. They are few in number; in fact, mountain-ranges form a less important feature of lunar than of terrestrial geography. On the other hand, the lunar ring-mountains and craters far exceed those of our earth in size and importance. The large craters may, in fact, be regarded as characteristic features of lunar scenery. There are several craters exceeding 100 miles in diameter. It is strange to consider that though the ringed wall surrounding some of these larger craters exceeds 10,000 feet in height, no trace of the highest peaks of such a wall would be visible from the middle of the enclosed plain. Conversely, an observer standing on one of the highest peaks beside one of these great craters, would not see half the floor of the crater, while more than half the horizon line around him would belong to the enclosed plain, and would appear as level as the horizon seen from a height overlooking a great prairie. These ringed plains and larger craters seem to belong to the third great era of the moon's history. The bright high regions and dark low levels called seas must have been formed while the greater part of the crust was intensely hot. The contraction of the cooling crust on the nucleus, which cooled far less slowly, led to the formation of the great ray systems. But though such systems extend from great craters, these craters themselves probably attained their present form far later. The crust having in great part cooled, the nucleus began in turn to shrink more quickly than the crust, having more heat to part with. Thus the crust, closing in upon the shrinking nucleus, formed the corrugations and wrinkles which can be seen under telescopic scrutiny in nearly every part of the visible lunar surface. The process was accompanied necessarily by the development of great heat—the thermal equivalent of the mechanical process of contraction. Mallet has shown that the process of contraction at present occurring in the earth's crust gives rise tothe greater part of the heat to which volcanic phenomena are due. If this is so in the earth's case at present, how tremendous must have been the heat evolved by the far more rapid contraction of the moon's mass in the remote era we are considering, when probably her heat passed into space unchecked by the action of a dense moisture-laden atmosphere! We can well understand that enormous volumes of heated gas would be formed—including steam, for there is good reason to believe that water is present in large quantities in the moon's interior. The imprisoned gas would find an outlet at points of least resistance, the centres, namely, of the great radiating systems of streaks. These centres would certainly be regions of outlet. But they would not be sufficient. We can understand then why every ray system extends from a great crater, though that crater was really formed after the system of radiating streaks; and we can equally understand why these central craters are not the only or even the chief of the great craters in the moon. Here again I would suggest that possibly the careful study of American geology might disclose features illustrating the great lunar craters.

When we pass to the smaller craters, ranging in diameter from seven or eight miles to less than a quarter of a mile, even if there be not some far smaller and beyond the range of the most powerful telescopes man can construct, we find ourselves among objects resembling those with which the study of our own earth has rendered us familiar. When Sartorius's map of Etna and the surrounding region was first seen at the Geological Society's rooms, many supposed that it represented lunar features. The Vesuvian volcanic region, again, is presented side by side with a lunar region of similar extent in Nasmyth's fine treatise on the moon, and the resemblance is very close. Considering the part which water plays in producing terrestrial volcanic phenomena, it may reasonably be doubted whether there is in reality so close a resemblance as a superficial comparison (and we can make no other) would suggest. There are those, indeed, who believe that some of the multitudinous small craters of the moon have had their origin in the downfall of meteoric masses on her once plastic surface; and strange though the thought may seem, there would be considerable difficulty in showing how the surface of the moon could have remained without traces of the meteoric downfall to which during myriads of centuries she was exposed undefended by that atmospheric shield which protects our earth from millions of meteors yearly falling upon her. We could only attribute the smallest lunar craters, however, to this cause. It may be noticed in passing that Professor Newcomb, apparently referring to this suggestion, which some had thought too fanciful to be seriously advanced, says that 'the figures of these inequalities (the small craters) can be closely imitated by throwing pebbles upon the surface of some smooth plastic mass, as mud or mortar.' Craters, however, larger than a mile or so in diameter, and many also of smaller dimensions, must be regarded as due to the same process of contraction which produced the great craters, but as belonging to an era when this process went on less actively. In like manner another feature of the moon's surface, the existence of narrow furrows calledrilles, which sometimes extend to a considerable distance, passing across levels, intersecting crater walls, and reappearing beyond mountain-ranges as though carried under like tunnels, must be regarded as due to the cracking of the crust thus slowly shrinking.

It is noteworthy that the signs of change which have been suspected during recent years belong to these smaller and probably more recent lunar formations. In November, 1866, Dr. Schmidt, chief of the Athens Observatory, announced that the crater Linné in the lunar Sea of Serenity was missing. To understand the importance of this announcement, let it simply be noted that the quantity of matter necessary to fill that crater up would be at least equal to that which would be required to form a mountain covering the whole area of London to a height of two miles! The crater was described by former lunarobservers as at least five miles in diameter and very deep. It is not now actually missing, as Schmidt supposed, but it is certainly no longer deep. It is, in fact, exceedingly shallow. Sir J. Herschel's opinion was that the crater had been filled up from beneath by an effusion of viscous lava, which, overflowing the rim on all sides, poured down the outer slope so as to efface its ruggedness and convert it into a gradual declivity casting no stray shadows. But the stupendous nature of the disturbing forces necessary to produce such an overflow of molten matter has led most astronomers to adopt in preference the theory that the wall surrounding the crater has been overthrown, either in consequence solely of the processes of contraction and expansion described above, or from the reinforcement of their action by the effects due to sublunarian energies. Some consider that the descriptions of the crater by Mädler and Lohrmann (which slightly differ) were erroneous, and that there has been no real change. Others deny that any change has occurred, on the ground that Linné varies in aspect according to the manner of its illumination. This I perceive is Professor Newcomb's explanation, who considers such variations 'sufficient to account for the supposed change.' But since the time of Schmidt's announcement Linné has several times been observed under nearly the same conditions as by Mädler and Lohrmann, as the great shadows formerly seen in its interior have not reappeared. There seems to be great reason for believing that a change has really occurred there.

The discovery announced by Dr. Klein is of a different nature. Near the middle of the visible half of the moon there is a well-known though small crater called Hyginus, the neighbourhood of which has been often and carefully examined. While examining this part of the moon's surface with an excellent 5-1/2in. telescope, in May, 1877, Dr. Klein observed a small crater full of shadow, and apparently nearly three miles in diameter. It formed a conspicuous object on the Sea of Vapours. Having frequentlyobserved this region during the last few years, he felt certain that no such crater existed there in 1876. He communicated his discovery to Dr. Schmidt, who stated, in reply, that in all the numerous drawings he had made of this lunar region no such crater was indicated. It is not shown in the great chart by Beer and Mädler, or in Lohrmann's map. Further observation showed that the crater is a deep, conical opening in the moon's surface. Soon after the sun has risen at that part of the moon, and, as later observations confirm, shortly before sunset there, the opening is entirely in shadow, and appears black. But when the sun is rather higher it appears grey, and with a yet higher sun it can no longer be distinguished. It can, however, be seen when the sun is very high on that part of the moon, appearing then somewhat brighter than the surrounding region, a circumstance which does not hitherto seem to have been noticed by either Klein or Schmidt.

The moon's surface has been so long and so carefully studied, that it is almost impossible to understand how such a crater as now certainly exists in the Sea of Vapours near Hyginus could have escaped detection. Craters of the kind exist, indeed, in hundreds on the moon's surface. But many astronomers have given years of their life to the study of such objects; and the centre of the moon's disc, for reasons which astronomers will understand, has been studied with exceptional care. It seems so unlikely that a deep crater three miles in diameter could escape recognition, that some astronomers have not hesitated to regard the newly-detected crater as certainly a new formation. For my own part, though it seems almost impossible to explain how such a crater could have remained so long unnoticed, I can regard the evidence of change as amounting only to extreme probability so far as it depends on the result of past telescopic scrutiny of the moon.

Admitting that a change had occurred, it would not follow that it had been produced by volcanic forces. It seems far more likely that a floor originally covering the conical hole now existing in the Sea of Vapours has given way at last under the effect of long-continued processes of expansion and contraction, which would operate with special energy over a region, like the Sea of Vapours, near the moon's equator.

But there remains to be mentioned a form of evidence respecting lunar features which could not be effectively applied to the case of the crater Linné, because the moon had only been subject to the necessary method of examination during a few years before that crater was missed. I refer to lunar photography. Many objects less than two miles in diameter are shown in the best photographs of our satellite by Rutherfurd, De la Rue, Ellery, and Draper; and as the moon has been photographed in every phase, some among the views might fairly be expected to show Klein's crater if it really existed before 1877. I do not find that in any lunar photographs the crater is shown as a black or dark gray spot. But in Rutherfurd's splendid photograph of the moon on March 6, 1865 (when the moon was about nine days five hours old), the place of Klein's crater is occupied by a small spot lighter than the surrounding 'sea.' This is the usual appearance of a small crater under a high sun; and though it may indicate only the existence of a flat crater floor in 1865 where now a great conical hole exists, it throws some degree of doubt on the occurrence of any change at all there. The case strongly suggests the necessity for continuing the work of lunar photography, which seems of late years to have flagged. Photographs of the moon should be taken in every aspect and in every stage of her librational swayings. Possessing such a series, we should be able to decide at once whether any newly-recognised crater was in reality a new formation or not.

THE NOVEMBER METEORS.

During November 13 and 14 the earth is passing through the region along which lies the course of the family of meteors called the Leonides, sometimes familiarly known as the November meteors. When at this time of the year the meteor region thus traversed by the earth is densely strewn with meteors, there occurs a display of falling stars, one of the most beautiful, and, rightly understood, one of the most remarkable of all celestial phenomena. Of old, indeed, when it was supposed that these meteors were purely meteorological phenomena, they were not thought specially interesting objects. They were held by some as mere weather-portents. It was only when a storm of wind was approaching,vento impendente, according to Virgil, that a shower of meteors was to be seen. Gross ignorance, indeed, has given to showers of falling stars an interest surpassing even that which has become attached to them through the discoveries of modern science, for they have been regarded as portending the end of the world. The shower of November 13, 1833, which was seen in great splendour in America, frightened the negroes of the Southern States nearly out of their wits. A planter of South Carolina relates that he was awakened by shrieks of horror and cries for mercy from 600 or 700 negroes. When he went out to see what was the matter, he found the negroes prostrate on the ground, 'some speechless, some with bitterest cries imploring God to spare the world and them.'There is, however, a grandeur in the interpretation placed by modern science upon these beautiful displays which dwarfs into littleness even such ideas as have been suggested by the terrors of superstition. We perceive that meteors are not mere terrestrial phenomena, nor of brief existence. They speak to us of domains in space compared with which the volume of our earth—nay, even the volume of the sun himself—is a mere point: of time-intervals compared with which the millions of years spoken of by geologists appear but as mere seconds.

The special meteor family whose track the earth crosses on November 13-14 forms a mighty ellipse round the sun, extending more than 19 times farther from him than the track of our earth, which yet, as we know, lies more than 92,000,000 miles from the sun. Along this tremendous orbit the meteors speed with planetary but varying velocity, crossing the track of our earth with a velocity exceeding by more than a third her own swift motion of about 19 miles in every second of time. Coming down somewhat aslant, but otherwise meeting the earth almost full tilt, the meteors rush into our air at the rate of more than 40 miles per second. They are so intensely heated as they rush through it that they are turned into the form of vapour, insomuch that we never make acquaintance with the members of this particular meteoric family in the solid form. In this respect they resemble the greater number of our meteoric visitants. It is, indeed, a somewhat fortunate circumstance for us that this is so, for if Professor Newton, of Yale College (United States), is right in estimating the total number of meteors, large and small, which the earth encounters per annum at 400,000,000, it would be rather a serious matter if all or most of these bodies were not warded off. The least of them, even though a mere grain perhaps in weight, would yet, arriving with planetary velocity exceeding a hundredfold or more the velocity of a cannon-ball, prove an awkward missile if it struck man or animal. But the air effectually saves us from all save a few fire-balls which are large enough toremain in great part solid until they actually strike the earth itself.

The importance of the meteors in the planetary system will be recognised when we remember that the November group alone extends along its oval course in one complete system of meteors for a length of more than 1,700 millions of miles, with an average thickness of about a million miles (determined by noting the average time occupied by the earth in passing through the system on November 13-14), and an unknown cross breadth which probably does not fall short of three or four millions of miles. Other systems are, no doubt, far more important, for it has been found that meteors follow in the track of comets. Now the November meteors follow in the track of a comet (Tempel's comet of 1866), which was so small when last favourably placed for observation that it escaped detection by the naked eye. If so small a comet as this is followed by so large a meteoric system, in which also meteors are strewn so richly that during the passage of the earth through it, tens of thousands of meteors have been counted, how vast must be the numbers and how large probably the individual bodies following in the track of such splendid comets as Newton's, Donati's (1858), the comets of 1811, 1847, 1861, and others! For it should be remembered that we become cognisant of the existence of a meteoric system only when the earth threads its way through one, when those which she encounters may become visible as falling stars if it so chances that she encounters them on the dark or night half of her surface. But the earth is far smaller compared with a system like the November meteor-flight than a rifle-ball compared with the largest flight of birds ever yet seen. Such a ball fired into a very dense and widely extending flock of birds might encounter here and there along its course some five or six birds—not one in 10,000, perhaps, of the entire flight; and if the flock continued flying with unchanging course, a hundred rifle balls might be fired through it without seemingly reducing its numbers. Ourearth has passed hundreds of times through the November meteor system, yet its meteoric wealth has scarcely been reduced at all, so exceedingly minute is the track of the earth through the meteor system compared with the extension of the system itself. The region through which the earth has passed is less than a billionth part of the entire region occupied by the system. But the November system is but one among several hundreds through which the earth passes—in other words, the systems which chance to be traversed by that mere thread-like ring in space traversed each year by the earth, are not a millionth, not a billionth, of the total number of such systems. It will be conceived, therefore, that the total amount of meteoric matter, travelling on orbits of all degrees of eccentricity and extension from the sun and inclined at all angles to the general plane of the solar system, must be enormously great. The idea once advanced by an eminent astronomer that the total quantity of unattached matter, so to speak, existing within the solar domain must be estimated rather by pounds than by tons is now altogether exploded. It would be truer to say that the totality of matter thus freely travelling around the sun must be estimated by billions of tons rather than by millions.

Whether it is likely that there will be a display of meteors to-night (or, rather, to-morrow morning), is a question to which most astronomers would be disposed, we believe, to reply definitely in the negative. The display of November 13-14, 1866, was very brilliant; that of 1867 (best seen in the United States) was almost equally so; but successive showers steadily diminished. In other words, the part of the system crossed by the earth in 1866 and 1867 was very rich, but the part which she crossed afterwards (the rich part having passed far on towards the remote aphelion of the system outside the orbit of Uranus) was less rich. For the last few years very few November meteors have been seen, though the few stragglers which have been seen, and have been identified as belonging to the family by their pathsathwart the star-depths, have been almost as interesting to astronomers as the showers of such bodies seen in 1799, 1833, 1866, and 1867. But it is not altogether impossible that in the small hours 'ayont the twal' to-morrow morning a shower of meteors may be seen. For Schiaparelli (the Italian astronomer who first started the ideas which led when properly followed up to the discovery of the relations existing between meteors and comets) asserts that it has happened before now that the November meteors have appeared in great numbers in years lying midway between the times ofmaximumdisplay. These times are separated on the average by about 33¼ years. Thus, in 1799, there was a great display of November meteors, a shower rendered specially celebrated by Humboldt's description. In 1833 there was another, the display which so terrified the negroes of South Carolina, but more interesting scientifically as described by Arago. In 1866 the shower again attained itsmaximumsplendour, though the display of 1867 was little inferior. It will not be till 1899 that another great shower of November meteors may be confidently looked for. But if Schiaparelli be right, it is quite possible that there may be a shower this year, due to some scattered flight of the November meteors which, delayed accidentally (through some special perturbation) many hundreds of years ago, has come in the course of ages to travel nearly half a circuit behind the richest part of the system, the 'gem of the meteor-ring,' as it has been poetically called. Even, however, though no display of November meteors should be seen, yet the recognition of even a few scattered stragglers would be exceedingly interesting to astronomers. A single meteor seen to-night which could be regarded as certainly belonging to the November system would suffice to show the possibility that a whole flight of the November meteors might travel at a similar distance behind the main body. It would be more easy, however, to identify two such meteors than one, six than two, and a score than half-a-dozen. The only way in which a meteor can be questioned, so to speak, respectingthe family it belongs to, is by noting its path across the sky. If this path tends directly from the constellation Leo (however remote Leo may be from the part of the heavens traversed by the meteor), the chances are that the meteor is a Leonid, or one of the November family. If the path tends from that particular part of the constellation Leo (near the end of the curved blade of the so-called Sickle in Leo), the probability of the meteor being a Leonid is increased. If two or more meteors are seen to-morrow morning (after 12.30) which both tend from the Sickle in Leo, even though they seem to tend in opposite directions, the chances are yet greater that they are travelling in parallel paths along the track of the November meteors, but some 2,000 million miles behind the main body. Should the number mount up to a score or so, the conclusion would be, to all intents and purposes, certain; and the possible occurrence of even a shower of Leonids at a time midway between the customarymaximaof the meteoric displays would be placed beyond question.

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