The able and original researches of the celebrated Reichenbach, who has made meteoric phenomena the subject of long-continued and enthusiastic investigation, have attracted the general attention of scientific men. It is proposed to present, in the following chapter, a briefresuméof his views and conclusions.
1.The Constitution of Comets.—It is a remarkable fact that cometary matter has no refractive power, as is manifest from the observations of stars seen through theirsubstance.20These bodies, therefore, are not gaseous; and the most probable theory in regard to their nature is that they consist of an infinite number of discrete, solid molecules, at great distances from each other, with very little attraction among themselves, or toward the nucleus, and having, therefore, great mobility. Now Baron Reichenbach, having carefully examined a great number of meteoric stones, has found them for the most partcomposed of extremely minute globules, apparently cemented together. He hence infers that they have been comets—perhaps very small ones—whose component molecules have by degrees collected into single masses.
2.The Number of Aerolites.—The average number of aerolitic falls in a year was estimated by Schreibers, as previously stated, at 700. Reichenbach, however, after a thorough discussion of the data at hand, makes the number much larger. He regards the probable annual average, for the entire surface of the earth, as not less than 4500. This would give about twelve daily falls. They are of every variety as to magnitude, from a weight of less than a single ounce to over 30,000 pounds. The Baron even suspects the meteoric origin of large masses of dolerite which all former geologists had considered native to our planet. In view of the fact that from the largest members of our planetary system down to the particles of meteoric dust there is an approximately regular gradation, and that the larger, at least in some instances, appear to have been formed by the aggregation of the smaller, he asks may not the earth itself have been formed by an agglomeration of meteorites? The learned author, from the general scope of his speculations, would thus seem to have adopted a form of the nebular hypothesis somewhat different from that proposed by Laplace.
3.Composition and mean Density of Aerolites.—A large proportion of meteoric stones are similar in structure to the volcanic or plutonic rocks of the earth; andallconsist of elements identical with those in our planet's crust. Their mean density,moreover, is very nearly the same with that of the earth. These facts are regarded by Reichenbach as indicating that those meteoric masses which are daily becoming incorporated with our planet, have had a common origin with the earth itself. Baron Reichenbach's views, as presented by himself, will be found at length inPoggendorf's Annalenfor December, 1858.
Stability of the Solar System.—The well-known demonstrations of the stability of the solar system, given by Lagrange and Laplace, are not to be accepted in an unlimited sense. They make no provision against the destructive agency of a resisting medium, or the entrance of matter into the solar domain from the interstellar spaces. In short, the conservative influence ascribed to these celebrated theorems extends only to the major planets; and even in their case it is to be understood as applying only to their mutual perturbations. The phenomena of shooting-stars and aerolites have demonstrated the existence of considerable quantities of matter moving inunstableorbits. The amount of such matter within the solar system cannot now be determined; but the term probably includes the zodiacal light, many, if not all, of the meteoric rings, and a large number of comets. These unstable parts of the system are being gradually incorporated with the sun, the earth, and doubtless also with the other large planets. It is highly probable that at former epochs the quantity of such matter was much greater than at present, and that, unless new supplies be receivedab extra, it must, by slow degrees, disappear from the system.
The fact, now well established, of the extensive diffusion of meteoric matter through the interplanetary spaces has an obvious bearing on Encke's theory of a resisting medium. If we grant the existence of such an ether, it would seem unphilosophical to ascribe to it one of the properties of a material fluid—the power of resisting the motion of all bodies moving through it—and to deny it such properties in other respects. Its condensation, therefore, about the sun and other large bodies must be a necessary consequence. This condensation existed in the primitive solar spheroid, before the formation of the planets: the rotation of the spheroid would be communicated to the coexisting ether; and hence,during the entire history of the planetary system, the ether has revolved around the sun in the same direction with the planets. This condensed ether, it is also obvious, must participate in the progressive motion of the solar system.
But again; even if we reject the doctrine of the development of the planetary bodies from a rotating nebula, we must still regard the density of the ether as increasing to the center of the system. The sun's rotation, therefore, would communicate motion to the first and denser portions; this motion would be transmitted outward through successive strata, with a constantly diminishing angular velocity. The motion of the planets themselves through the medium in nearly circular orbits would concur in imparting to it a revolution in the same direction. Whether, therefore, we receive or reject the nebular hypothesis, the resistance of the ethereal medium to bodies moving in orbits of small eccentricity andin the direction of the sun's rotation, becomes an infinitesimal quantity.
The hypothesis of Encke, it is well known, was based solely on the observed acceleration of the comet which bears his name. More recently, however, a still greater acceleration has been found in the case of Faye's comet. Now as the meteoric matter of the solar system is aknowncause for such phenomena, sufficient, in all probability, both in mode and measure, the doctrine of a resisting ethereal medium would seem to be a wholly unnecessary assumption.
An analysis of any extensive table of meteorites and fire-balls proves that a greater number of aerolitic falls have been observed during the months of June and July, when the earth is near its aphelion, than in December and January, when near its perihelion. It is found, however, that the reverse is true in regard to bolides, or fire-balls. Now the theory has been held by more than one physicist, that aerolites are the outriders of the asteroid ring between Mars and Jupiter; their orbits having become so eccentric that in perihelion they approach very near that of the earth. If this theory be the true one, the earth would probably encounter the greatest number of those meteor-asteroids when near its aphelion. The hypothesis therefore, it has been claimed, appears to be supported by well-known facts. The variation, however, in the observed number of aerolites may be readily accounted for independently ofany theory as to their origin. The fall of meteoric stones would evidently be more likely to escape observation by night than by day, by reason of the relatively small number of observers. But the days are shortest when the earth is in perihelion, and longest when in aphelion; the ratio of their lengths being nearly equal to that of the corresponding numbers of aerolitic falls.
On the other hand, it is obvious that fire-balls, unless of very extraordinary magnitude, would not be visible during the day. Theobservednumber will therefore be greatest when the nights are longest; that is, when the earth is near its perihelion. This, it will be found, is precisely in accordance with observation.
It has been found, moreover, that a greater number of meteoric stones fall during the first half of the day, that is, from midnight to noon, than in the latter half, from noon to midnight. This would seem to indicate that a large proportion of the aerolites encountered by the earth have direct motion.
Height of the Atmosphere.—The weight of a given volume of mercury is 10,517 times that of an equal volume of air at the earth's surface; and since the mean height of the mercurial column in the barometer is about thirty inches, if the atmosphere were of uniform density its altitude would be about 26,300 feet, or nearly five miles. The density rapidly diminishes, however, as we ascend above the earth's surface. Calling it unity at the sea level, the rate of variation is approximately expressed as follows:
From this table it will be seen that at the height of 35 miles the air is one thousand times rarer than at the surface of the earth; and that, supposing the same rate of decrease to continue, at the height of 140 miles the rarity would be one trillion times greater. The atmosphere, however, is not unlimited. When it becomes so rare that the force of repulsion between its particles is counterbalanced by the earth's attraction, no further expansion is possible. To determine the altitude of its superior surface is a problem at once difficult and interesting. Not many years since about 45 or 50 miles were generally regarded as a probable limit. Considerable light, however, has been thrown upon the question by recent observations in meteoric astronomy. Several hundred detonating meteors have been observed, and their average height at the instant of their first appearance has been found to exceed 90 miles. The great meteor of February 3d, 1856, seen at Brussels, Geneva, Paris, and elsewhere, was 150 miles high when first seen, and a few apparently well-authenticated instances are known of a still greater elevation. We conclude, therefore, from the evidence afforded bymeteoric phenomena, that the height of the atmosphere is certainly notlessthan 200 miles.
It might be supposed, however, that the resistance of the air at such altitudes would not develop a sufficient amount of heat to give meteorites their brilliant appearance. This question has been discussed by Joule, Thomson, Haidinger, and Reichenbach, and may now be regarded as definitively settled. When the velocity of a meteorite is known the quantity of heat produced by its motion through air of a given density is readily determined. The temperature acquired is the equivalent of the force with which the atmospheric molecules are met by the moving body. This is about one degree (Fahrenheit) for a velocity of 100 feet per second, and it varies directly as the square of the velocity. A velocity, therefore, of 30 miles in a second would produce a temperature of 2,500,000°. The weight of 5280 cubic feet of air at the earth's surface is about 2,830,000 grains. This, consequently, is the weight of a column 1 mile in length, and whose base or cross section is one square foot. The weight of a column of the same dimensions at a height of 140 miles would be about 1/350000th of a grain. Hence the heat acquired by a meteoric mass whose cross section is one square foot, in moving 1 mile would be one grain raised 7-1/7 degrees, or one-fifth of a grain 2500° in 70 miles. This temperature would undoubtedly be sufficient to render meteoric bodies brilliantly luminous.
But there have been indications of an atmosphere at an elevation of more than 500 miles. A discussion of the best observations of the great aurora seenthroughout the United States on the 28th of August, 1859, gave 534 miles as the height of the upper limit above the earth's surface. The aurora of September 2d, of the same year, had an elevation but little inferior, viz., 495 miles. Now, according to the observed rate of variation of density, at the height of 525 miles, the atmosphere would be so rare that a sphere of it filling the orbit of Neptune would contain less matter than 1/30th of a cubic inch of air at the earth's surface. In other words, it would weigh less than 1/90th of a grain. We are thus forced to the conclusion either that the law of variation is not the same at great heights as near the surface; or, that beyond the limits of the atmosphere of air, there is another of electricity, or of some other fluid.
Of the various theories proposed by astronomers to account for the origin of the sun's light and heat, only two have at present any considerable number of advocates. Theseare—
1.The Chemical Theory; according to which the light and heat of the sun are produced by the chemical combination of its elements; in other words, by an intense combustion.
2.The Meteoric Theory, which ascribes the heat of our central luminary to the fall of meteors upon its surface. The former is advocated with great ingenuity by Professor Ennis in a recent work on "The Origin of the Stars, and the Causes of their Motions and their Light." It has, on the other hand, been ably opposed by Dr. Mayer, Professor William Thomson, and other eminent physicists. A brief examination of its claims may not be destitute of interest.
If the sun's heat is produced by chemical action, whence comes the necessary supply of fuel to support the combustion? The quantity of solar heat radiated into space has been determined with at least an approximation to mathematical precision. We know also the amount produced by the combustion of a given quantity of coal. Now it has been found bycalculation that if the sun were a solid globe of coal, and a sufficient supply of oxygen were furnished to support its combustion, the amount of heat resulting from its consumption would be less than that actually emitted during the last 6000 years. In short,no knownelements would meet the demands of the case. But it is highly probable that the different bodies of the solar system are composed of the same elements. This view is sustained by the well-known fact that meteoric stones, which have reached us from different and distant regions of space, have brought us no new elementary substances. Thechemicaltheory of solar heat seems thus encumbered with difficulties well-nigh insuperable.
Professor Ennis' mode of obviating this objection, though highly ingenious, is by no means conclusive. The latest analyses of the solar spectrum indicate, he affirms, the presence of numerous elements besides those with which we are acquainted. Some of these may yield by their combustion a much greater amount of heat than the same quantity of any known elements in the earth's crust. "Every star," he remarks, "as far as yet known, has a different set of fixed lines, although there are certain resemblances between them. They lead to the conclusion that each star has, in part at least, its peculiar modifications of matter, called simple elements; but the number of stars is infinite, and therefore the number of elements must beinfinite."21He argues, moreover, that in a globe so vast as the sun there may be forces in operation with whose nature weare wholly unacquainted. This leaving of theknownelements as well as theknownlaws of nature forunknown possibilitieswill hardly be satisfactory to unbiased minds.
Again: that the different bodies of the universe are composed of different elements is inferred by our author from the following among other considerations: "In our solar system Mercury is sixty or eighty times more dense than one of the satellites of Jupiter, and probably in a much greater proportion denser than the satellites of Saturn. This indicates a wide difference between the nature of their elements." This statement is again repeated in a subsequentpage.22"The densities of the planets and their satellites prove that they are composed of very different elements. Mercury is more than sixty times, and our earth about fifty times, more dense than the inner moon of Jupiter. Saturn is only about one-ninth as dense as the earth; it would float buoyantly on water. There is a high probability that the satellites of Saturn and Uranus are far lighter than those of Jupiter. Between the two extremes of the attendants of the sun, there is probably a greater difference in density than one hundred to one; and from one extreme to the other there are regular gradations of small amount.
"The difference in constitution between the earth and the moon is seen in their densities: that of the moon being about half that of the earth. The nitrogen of our globe is found only in the atmosphere, and such substances as derive it from theatmosphere. The moon has no appreciable atmosphere, and therefore, in a high probability, no nitrogen."
The statements here quoted were designed to show that the physical constitution of the sun and planets is widely different from that of the earth, and that the combustion ofsomeof the elements in this indefinite variety may account for the origin of solar heat. Let us examine the facts.
According to Laplace the mass of Jupiter's first satellite is 0·000017328, that of Jupiter being 1. The diameter is 2436 miles. Hence the corresponding density is a little more thanone-fifthof the mean density of the earth. In other words, it is somewhat greater than the density of water, and very nearly equal to that of Jupiter himself. Professor Ennis' value is thereforeerroneous.23In regard to the densities of the Saturnian and Uranian satellites nothing is known, and conjecture is useless. In short, Saturn has the least mean density of all the planets, primary or secondary, so far as known. This may be owing to the great extent of his atmospheric envelope. The density of the moon is but three-fifths that of the earth: it is to be borne in mind, however, that themassandpressureare also much less.
With respect to meteorites the same author remarks that "like the moon, they are probably satellites of the earth; but being very small, they areliable to extraordinary perturbations, and hence strike the earth in many directions." Here, again, hisfactsare at fault; for (1) the observed velocities of these bodies are inconsistent with the supposition of their being satellites of the earth; and (2) the amount of perturbation of such bodies does not vary with their masses: asmallmeteorite would fall toward the earth or any other planet with no greater velocity than alargeone.
It has been shown in a previous chapter that immense numbers of meteoric asteroids are constantly traversing the planetary spaces—that many millions, in fact, daily enter the earth's atmosphere. Reasons are not wanting for supposing the numbers of these bodies to increase with great rapidity as we approach the center of the system. Moreover, on account of the greater force of gravity at the sun's surface the heat produced by their fall must be much greater than at the surface of the earth. It has been calculated that if one of these asteroids be arrested in perihelion by the solar atmosphere, the quantity of heat thus developed will be 9000 times greater than that produced by the combustion of an equal mass of coal. There can, therefore, be no reasonable doubt that aportionof the sun's heat is produced by the impact of meteoric matter. In considering the probability that it ischieflyso generated, the following questions naturally present themselves:
1.What amount of matter precipitated upon the sun would develop the quantity of heat actually emitted?—This question has been satisfactorily discussed by eminent physicists, and it will be sufficient for our purpose to give the result. According to Professor William Thomson, of Glasgow, the present rate of emission would be kept up by a meteoric deposit which would form an annual stratum 60 feet in thickness over the sun's surface.
2.Could such an increase of the sun's magnitude be detected by micrometrical measurement?—This inquiry is readily answered in the negative. The apparent diameter would be augmented only one second in 17,600 years.
3.Is there any known or visible source from which this amount of meteoric matter may be supplied?—Thomson, Mayer, and other distinguished writers regard the zodiacal light as the source of such meteorites. The inner portions of this immense "tornado" must be resisted in their motions by the solar atmosphere, and hence precipitated upon the sun's surface.
4.Would this increase of the sun's mass derange the motions of the solar system?—To this question Prof. Ennis gives an affirmative answer; his first objection to the theory under consideration being stated as follows: "The constant accumulation of such materials, during hundreds of millions of years, would increase the body of the sun and its consequent gravity so greatly as to derange the entire solar system, by destroying the balance between the centripetal and centrifugal forces now acting on theplanets."24This, it must be confessed, would be a valid objection, if the meteoric matter were supposedto be derived from the extra-planetary spaces. As their source, however,—the zodiacal light—is interior to the earth's orbit, it can have no application to any planet exterior to Venus. Most probably the greater portion of the meteoric mass is even within the orbit of Mercury, so that the effect of its convergence could scarcely be noticed even in the motion of the interior planets. In pre-historic time the zodiacal light may have extended far beyond the earth's orbit. If so, its convergence to its present dimensions was undoubtedly attended by an acceleration of the earth's mean motion. We can of course have no evidence that such a shortening of the year has never occurred.
The second objection urged against the meteoric theory by the author of "The Origin of the Stars" is thus expressed: "As we must believe that all stars were lighted up by the same means, so we must believe, according to this theory, that the present interior heat of the earth and its former melted condition in both exterior and interior, was caused by the fall of meteorites. But if so, they must have gradually ceased to fall, as space became cleared of their presence, and we would now find a thick covering of meteorites on the earth's cooled surface. Instead of this, we find them very rarely, and in accordance with their present very rare falls."
To this it may be replied that the primitive igneous fluidity of the earth and planets was a necessary consequence of their condensation—a fact which has no inconsistency with the theory in question.
A differentmechanicaltheory of the origin of solar heat is advocated by Professor Helmholtz in his interestingworkOn the Interaction of Natural Forces. In regard to the sun he says: "If we adopt the very probable view, that the remarkably small density of so large a body is caused by its high temperature, and may become greater in time, it may be calculated that if the diameter of the sun were diminished only the ten-thousandth part of its present length, by this act a sufficient quantity of heat would be generated to cover the total emission for 2100 years. Such a small change besides it would be difficult to detect by the finest astronomicalobservations."25The same view is adopted by Dr. Joel E. Hendricks, of Des Moines,Iowa.26
Having shown that meteor-asteroids are diffused in vast quantities throughout the universe; that according to eminent physicists the solar heat is produced by the precipitation of such matter on the sun's surface; and that Leverrier has found it necessary to introduce the disturbing effect of meteoric rings in order fully to account for the motion of Mercury's perihelion; we now propose extending the meteoric theory to a number of phenomena that have hitherto received no satisfactory explanation.
No theory as to the origin of the sun's light and heat would seem to be admissible unless applicable also to the sidereal systems. Will the meteoric theory explain the phenomena of variable and temporary stars?
"It has been remarked respecting variable stars, that in passing through their successive phases, they are subject to sensible irregularities, which have not hitherto been reduced to fixed laws. In generalthey do not always attain the same maximum brightness, their fluctuations being in some cases very considerable. Thus, according to Argelander, the variable star inCorona Borealis, which Pigott discovered in 1795, exhibits on some occasions such feeble changes of brightness, that it is almost impossible to distinguish the maxima from the minima by the naked eye; but after it has completed several of its cycles in this manner, its fluctuations all at once become so considerable, that in some instances it totally disappears. It has been found, moreover, that the light of variable stars does not increase and diminish symmetrically on each side of the maximum, nor are the successive intervals between the maxima exactly equal to each other."—Grant's History of Physical Astronomy, p. 541.
Of the numerous hypotheses hitherto proposed to account for these phenomena we believe none can be found to include and harmonize all the facts of observation. The theories of Herschel and Maupertius fail to explain the irregularity in some of the periods; while those of Newton and Dunn afford no explanation of the periodicityitself.27But let us suppose that among the fixed stars some have atmospheres of great extent, as was probably the case with the sun at a remote epoch in its history. Let us also suppose the existence of nebulous rings, like those of our own system, moving in orbits so ellipticalthat in their perihelia they pass through the atmospheric envelopes of the central stars. Such meteoric rings of varying density, like those revolving about the sun, would evidently produce the phenomena of variable stars. The resisting medium through which they pass in perihelion must gradually contract their orbits, or, in other words, diminish the intervals between consecutive maxima. Such a shortening of the period is now well established in the case ofAlgol. Again, if a ring be influenced by perturbation the period will be variable, like that ofMira Ceti. A change, moreover, in the perihelion distance will account for the occasional increase or diminution of the apparent magnitude at the different maxima of the same star. But how are we to account for the variations of brightness observed in a number of stars where no order or periodicity in the variation has as yet been discovered? It is easy to perceive that either a single nebulous ring with more than onehiatus, or several rings about the same star, may produce phenomena of the character described. Finally, if the matter of an elliptic ring should accumulate in a single mass, so as to occupy a comparatively small arc, its passage through perihelion might produce the phenomenon of a so-called temporary star.
Recent researches relating to nebulæ seem in some measure confirmatory of the view here presented. These observations have shown (1) a change of position in some of these objects, rendering it probable that in certain cases they are not more distant than fixed stars visible to the naked eye; and (2) a variationin the brilliancy of many small stars situated in the great nebula of Orion, and also the existence of numerous masses of nebulous matter in the form of tufts apparently attached to stars,—facts regarded as indicative of a physical connection between the stars and nebulæ.28
Besides thecosmicaltheory of aerolites which has been adopted in this work, and which is now accepted by a great majority of scientific men, at least four others have been proposed: (1) theatmospheric, according to which they are formed, like hail, in the earth's atmosphere; (2) thevolcanic, which regards them as matter ejected with great force from terrestrial volcanoes; (3) thelunar, which supposes them to have been thrown from craters in the moon; and (4) thesolarhypothesis, according to which they are projected by some tremendous explosive force from the great central orb of our system. The first and second have been universally abandoned as untenable. The third and fourth, however, are entitled to consideration.
The theory which regards meteoric stones as products of eruption in lunar volcanoes was received with favor by the celebrated Laplace: "As the gravity at the surface of the moon," he remarks, "is much less than at the surface of the earth, and as this body has no atmosphere which can oppose asensible resistance to the motion of projectiles, we may conceive that a body projected with a great force, by the explosion of a lunar volcano, may attain and pass the limit, where the attraction of the earth commences to predominate over that of the moon. For this purpose it is sufficient that its initial velocity in the direction of the vertical may be 2500 meters in a second; then in place of falling back on the moon, it becomes a satellite of the earth, and describes about it an orbit more or less elongated. The direction of its primitive impulsion may be such as to make it move directly toward the atmosphere of the earth; or it may not attain it, till after several and even a great number of revolutions; for it is evident that the action of the sun, which changes in a sensible manner the distances of the moon from the earth, ought to produce in the radius vector of a satellite which moves in a very eccentric orbit, much more considerable variations, and thus at length so diminish the perigean distance of the satellite, as to make it penetrate our atmosphere. This body traversing it with a very great velocity, and experiencing a very sensible resistance, might at length precipitate itself on the earth; the friction of the air against its surface would be sufficient to inflame it, and make it detonate, provided that it contained ingredients proper to produce these effects, and then it would present to us all those phenomena which meteoric stones exhibit. If it was satisfactorily proved that they are not produced by volcanoes, or generated in our atmosphere, and that their cause must be sought beyond it, in the regions of the heavens, the preceding hypothesis, whichlikewise explains the identity of composition observed in meteoric stones, by an identity of origin, will not be devoid of probability."—Système du Monde, t. ii. cap. v.
Knowing the masses and volumes of the earth and moon, it is easy to estimate the force of gravity at their surfaces, the distance from each to the point of equal attraction, and the force with which a projectile must be thrown from the lunar surface to pass within the sphere of the earth's influence. It has been calculated that an initial velocity of about a mile and a half in a second would be sufficient for this purpose—a force not greater than that known to have been exerted by terrestrial volcanoes. Thepossibility, therefore, that volcanic matter from our satellite may reach the earth's surface seems fairly admissible.
Since the time of Laplace, several distinguished European astronomers have regarded the lunar hypothesis as more or less probable. It was advocated as recently as 1851 by the late Prof. J. P. Nichol, of Glasgow. This popular and interesting writer, after describing Tycho, a large and well-known lunar crater, from which luminous rays or stripes radiate over a considerable part of the moon's surface, expresses the opinion that that immense cavity was formed by a single tremendous explosion. "Reflecting," he remarks, "on the probable suddenness and magnitude of that force, or rather of thatexplosiveenergy one of whose acts we have traced, as well as on the immense mass of matter which seems to have been thus violently dispersed, is not the inquiry a natural one,where is that matter now? It is a massindeed which cannot well have wholly disappeared. It filled a cavern 55 miles in breadth, and 17,000 feet deep—a cavern into which even now one might cast Chimborazo and Mont Blanc, and room be left for Teneriffe behind! Like rocks flung aloft by our volcanoes, did this immense mass fall back in fragments to the surface of the moon, or was the expulsive force strong enough to give it an outward velocity sufficient to resist the attractive power of its parent globe? The moon, be it recollected, is very small inmasscompared with the earth, and her attractive energy greatly inferior accordingly. Laplace has even calculated that the force urging a cannon-ball, increased to a degree quite within the limits of what is conceivable, could effect a final separation between our satellite and any of its component parts. It ispossiblethen, and, although not demonstrable, very far from a chimera, that the disrupted and expelled masses were, in the case of which we are speaking, driven conclusively into space; but if so, where are they now? where their new residence, and what their functions? In the emergency to which I refer, such fragments would necessarily wander among the interplanetary spaces in most irregular orbits, and chiefly in the neighborhood of the moon and the earth. Now, while the planetary orbits are so nicely adjusted that neither confusion nor interference can ever occur, it is not at all likely that the same order could be established here; nay, it is next to certain, that in the course of its orbital revolution our globe would ever and anon come in contact with these lunar fragments; in other words,STONESwould fall occasionally to its surface, and apparentlyfrom its atmosphere."—Planetary System, pp. 301, 302.
We have preferred to give the views of these eminent scientists in their own language. Olbers, Biot, and Poisson, who adopted the same theory, estimated theinitialvelocity which would be necessary in order that lunar fragments might pass the point of equal attraction, and also thefinal, or acquired velocity on reaching the earth's surface. The several determinations of the former were as follows:
The mean being almost exactly a mile and a half. The velocity on reaching our planet, according to Olbers, would be about six and a half miles. At the date of these calculations, however, the true velocity of aerolites had not been in any case satisfactorily determined. Since that time it has been found in numerous instances to exceedtwenty miles a second—a velocity greater than that of the earth's orbital motion. This fact of itself would seem fatal to the theory of a lunar origin.
At the meeting of the American Association for the Advancement of Science, in 1859, Dr. B. A. Gould read a paper on the supposed lunar origin of aerolites, in which the hypothesis was subjected to the test of a rigid mathematical analysis. We will not attempt even an abstract of this interesting memoir. It amounts, however, to a virtual disproof of the lunar hypothesis.
The theory which ascribes a solar origin to meteorites is not of recent date, having been held by Diogenes Laertius and other ancient Greeks. Among the moderns its advocates have been much less numerous than those of the lunar hypothesis. The late Professor Charles W. Hackley, of New York, regarded shooting-stars, aerolites, and even comets, as matter projected with enormous force from the solar surface. The corona seen during total eclipses of the sun he supposed to be the emanations of this matter through the intervals of the luculi.—(See the Proceedings of the American Association for the Advancement of Science, Fourteenth Meeting, 1860.) An ingenious theory, differing in its details from that of Professor Hackley, though somewhat similar in its general features, has lately been advocated by Alexander Wilcocks, M.D., of Philadelphia, in a memoir read before the American Philosophical Society, May 20th, 1864, and published in their Proceedings. In regard to this hypothesis it seems sufficient to remark that it fails to give a satisfactory account of the annual periodicity of meteoric phenomena.
Until about the middle of the present century the rings of Saturn were universally regarded as solid and continuous. The labors, however, of Professors Bond and Pierce, of Cambridge, Massachusetts, as well as the more recent investigations of Prof. Maxwell, of England, have shown this hypothesis to be wholly untenable. The most probable opinion, based on the researches of these astronomers, is, that they consist of streams or clouds of meteoric asteroids. The zodiacal light and the zone of small planets between Mars and Jupiter appear to constitute analogousprimaryrings. In the latter, however, a large proportion of the primitive matter seems to have collected in distinct, segregated masses. These meteoric zones have probably presented—what are not elsewhere found in the solar system—cases of commensurability in the planetary periods. The interior satellites of Saturn are so near the ring as doubtless to exert great perturbative influence. Unfortunately, the elements of the Saturnian system as determined by different astronomers are somewhat discordant. This, however, is by no means surprising when we consider the great distance of theplanet and the small magnitude of some of the satellites. For convenience of reference the mean apparent distances of the satellites, together with their periodic times, are given in the following table. The former are taken from Hind'sSolar System; the latter from Herschel'sOutlines of Astronomy.
TABLE I.—The Satellites of Saturn.
The late Professor Bessel devoted much attention to the theory of Titan, whose mean distance he found to be 20·706 equatorial radii of the primary. Struve's measurements of the ring are given in the second column of the following table. Sir John Herschel, however, regards the Russian astronomer's interval between the rings as "somewhat toosmall."29This remark is confirmed by the measurements of Encke, whose results are given in column third. The fourth contains themeanof Struve's and Encke's measurements; and the fifth, the same, expressed in equatorial radii of Saturn.
TABLE II.—The Rings of Saturn.
The interval, therefore, occupies precisely the space in which the periods would be commensurable with those of the four members of the system immediately exterior. Particles occupying this portion of theprimitivering would always come into conjunction with one of these satellites in the same parts of their orbits. Such orbits would become more and more eccentric until the matter moving in them would unite near one of the apsides with other portions of the ring.We have thus a physical cause for the existence of this remarkable interval.
The mean distances of the minor planets between Mars and Jupiter vary from 2·20 to 3·49. The breadth of the zone is therefore 20,000,000 miles greater than the distance of the earth from the sun; greater even than the entire interval between the orbits of Mercury and Mars. Moreover, theperiheliondistance of some members of the group exceeds theapheliondistance of others by a quantity equal to the whole interval between the orbits of Mars and the earth. TheOlbersianhypothesis of the origin of these bodies seems thus to have lost all claim toprobability.30Professor Alexander's theory of the disruption of a primitive discoidal planet of great equatorial diameter, is less objectionable; still, however, it requires confirmation. But whatever may have been the original constitution of thering,31its existence in its present form for an indefinite periodis unquestioned. Let us then consider some of the effects of its secular perturbation by the powerful mass of Jupiter.
Portions of the ring in which the periods of asteroids would be commensurable with that of Jupiter.—The breadth of this zone is such as to contain several portions in which the periods of asteroids would be commensurable with that of Jupiter. As in the case of the perturbation of Saturn's ring by the interior satellites, the tendency of Jupiter's influence would be to form gaps or chasms in the primitive ring.
For the purpose of facilitating the comparison of these numbers with the mean distances of the asteroids and of observing whether any order obtains in the distribution of these mean distances in space, we have arranged the minor planets, in the following table, in the consecutive order of their periods:
Periods and Distances of the Asteroids.
1. The first two members of the group, Flora and Ariadne, have very nearly the same mean distance. Immediately exterior to these, however, occurs a wide interval, including the distance at which seven periods of an asteroid would be equal to two of Jupiter.
2. On theouterlimit of the ring Freia, Cybele, and Sylvia have also nearly equal distances, and are separated from the next interior member by a wide space including the distance at which two periods would be equal to one of Jupiter, and also that at which five would be equal to one of Saturn.
3. Besides these extreme members of the group, our table contains eighty-six minor planets, all of which are included between the distances 2·26 and 3·16; the mean interval between them being 0·0105. The distances are distributed as follows:
The clustering tendency is here quite apparent.
4. The three widest intervals between these bodiesare—
and these, it will be observed, are the three remaining distances, indicated on a previous page, at which the periods of the primitive meteoric asteroids would be commensurable with that of Jupiter. Now, if the original ring consisted of an indefinite number of separate particles moving with different velocities, according to their respective distances, those revolving at the distance 2·4935—in the interval between Thetis and Hestia—would make precisely three revolutions while Jupiter completes one. A planetary particle at this distance would therefore always come in conjunction with Jupiter in the same parts of its path: consequently its orbit would become more and more eccentric until the particle itself would unite with others, either exterior or interior, thus forming an asteroidal nucleus, while the primitive orbit of the particle would be left destitute of matter, like the interval in Saturn's ring.
5. If the distribution of matter in the zone was originally nearly continuous, as in the case of Saturn's rings, it would probably break up into a number of concentric annuli. On account, however, of the great perturbations to which they were subject, these narrow rings would frequently come in collision. After their rupture, and while the fragments were collecting in the form of asteroids, numerous intersections of orbits and new combinations of matter would occur, so as to leave, in the present orbits, but few traces of the rings from which the existing asteroids were derived. A comparison, however, of the elements of Clytie and Frigga shows a striking similarity; and Professor Lespiault has pointed out a corresponding likeness between the orbits of Fidesand Maia. For these four asteroids the nodal lines and also the inclinations are nearly the same; while the periods differ by only a few days. It is probable, therefore, that they are all fragments of the same narrow ring. Finally, as they all move nearly in the same plane, they must at some future time approach extremely near each other, and perhaps become united in one large asteroid.