For comets larger, and which travel to greater distances, than those alluded to above, it is very difficult to get data on which we can form satisfactory calculations of the lengths of their orbits and mean velocities of revolution, for there is almost always awanting some one or more of their elements, or totally different statements given of their value; but we think we have found a few from which we can collect data sufficiently accurate to enable us to show that there is no necessity for going beyond the domains of the sun, asdescribed by us in a former chapter, to account for any one of the comets which have been taken notice of in astronomical history; and still less necessity to suppose that any of them have wandered, or been shot forth, from some neighbouring star into the solar system.
From the data we have been able to collect it would appear that when a comet comes to have a period of over 70 years, it is either too far removed from the sun at its aphelion passage, or its mass is too great for it to be perturbed by the attraction of any of the planets. For instance, we have Halley's comet, which has been observed for not far from 2000 years, whose period has averaged very close upon 77 years during the whole of that time, showing that it has not been perturbed to any appreciable extent when near its perihelion passage. No doubt 2000 years is a very small period of time to judge from, and its aphelion distance being only 3,258,000,000 miles, it might be influenced to some extent by some planet, so we can hardly count upon its being permanently exempt from perturbation. Indeed, Halley himself supposed that its velocity of revolution had been considerably increased when it was in the neighbourhood of Jupiter in the interval between 1607 and 1682; but if it was so, there must be some counter-perturbation which restores the balance so as to make the average period of 77 years. Looking over the register of its appearances, we find that in its re-appearances of the years 66 and 1758, the period was about 75 years, and that in those of 451 and 1066 it was 79 years; so that if there are perturbations, we must claim that there are also compensations. Seeing, then, that we can find no evidence to the contrary, we may suppose that when the periods of comets, and, perhaps more especially, when their aphelion distances reach to beyond—and the farther the more so—the orbit of the most distant planet, they may be looked upon as not being liable to be seriously perturbed by any of the members of the solar system, until something to the contrary had been proved. Following this idea, then, it occurs to us that something may be learnt from their mean velocities in their orbits, as will be seen from the following very small list of those we have been able to submit to calculation, which form the accompanying
TABLE XI.—Showing the Mean Velocities in Orbit of several Comets.
Theseorbital mean velocities per second have been calculated from aphelion distances as diameters and from circular orbits, which probably give results rather lower than would be derived from elliptical orbits—were they known—but on the other hand, the perihelion distances have not been taken into account in fixing the diameters—because they were unknown—so the error will be so far compensated, if not altogether.
We know that the mean velocities in orbit of the planets decrease as their distances from the sun increase, and our table, as far as it goes, leads us to believe that the same holds good with comets whose aphelion distances are comparable to those of the planets, in being measured by hundreds of years or less of revolution; but with those whose periods are measured by thousands of years, the same rule seems to fail. One thing, however, that we seem entitled to believe is that, generally speaking, the greater the period of revolution of a comet is, the less will be its mean velocity per second in its orbit. It will be observed that the average mean velocity of the three remote comets in the table is only 0·83 mile per second, and it is by no means unreasonable to suppose that the average mean velocity per second of any number of comets whose aphelion distances are greater than the highest of those in the table, is not likely to be so great as the average of thethree; on this understanding, then, let us take, or suppose, one whose mean velocity in orbit per second is only one mile, and look into what may be learnt from it.
Going back to the peak of α Geminorum which we supposed, atpage 321, to be condensed to 129,000 million miles in diameter of base, its height 1¼ billion miles, and distance from the sun 11 billion miles, we may take a comet formed from it as an example. If, then, we suppose the leading part of it to have been formed into a comet with that aphelion distance—11 billion miles—and other dimensions suitable to its new condition; taking its mean velocity in orbit at 1 mile per second, we find that its period of revolution might be 1,200,000 years, or three times greater than that of the comet of 1882, namely 400,000 years, mentioned by Mr. Chambers as being not very reliable, probably because its angles in orbit could not be measured with sufficient accuracy. Then, when we think that the sphere of the sun's attraction in that direction—of α Geminorum—extends to 67 billions of miles, and that there are stars more than 6 times farther off, e.g. Canopus, seeTable VII., we see that a supposed comet might have an aphelion distance equal to that; and were we further to consider that were its major axis 67 billion miles long, including aphelion and perihelion distances, and that it went straight from the one end of it to the other and back again, its period of revolution, if it could be so called, would be 8,500,000 years; that is 20 times greater than Mr. Chambers's doubtful 400,000 years for the comet of 1882. There seems, therefore, to be no necessity for the solar system sending its cometary produce to a foreign market; and our mechanical imagination is not sufficiently vivid to allow us to conceive what kind of potential energy even Jupiter can have to give an impetus to a comet, great enough to send it flying to so great a distance. What velocity would it have when it left the sun? And what would remain in it to carry it over the debatable land between the sun and a distant neighbour? Or are we to believe that all the solar system's produce of that kind is only sent over the channel, as it were, to our nearest neighbour, α Centauri? Conceptions of that kind aretoo elevated for us, and we must leave them alone. Mr. Chambers expresses doubts as to the determination of whether the orbit of a comet is elliptical or parabolic when its period of revolution is measured by hundreds of thousands of years, and we think we are safe in following him until actual proofs are presented. If the comet of 1882 never comes back, we may then believe it has gone elsewhere.
Having used up all the nebulous matter in the sun's domains, as described at the beginning ofChapter XV., or at least shown how it may have been, or may yet be, used up, we have now only to make a few remarks to prove that our description of the said domains is not by any means fanciful. It matters very little whether the solar system was begun to be brought into existence at the same time as the surrounding systems or before or after them. What is certain is that the sun's sphere of attraction among its neighbours is bounded, at the present time, just in the way we have taken to describe its domains. How they were filled with cosmic matter may be disputed, but filled they must have been somehow, if the solar system was formed out of a nebula; and the way adopted by us was the only one that occurred to us when we began to reconstruct the original nebula. Since then we have had time to reflect on our work, and to see how it points out the simplest way that can be conceived, which may be expressed in the few following words. We may suppose that the ether was the primitive matter, as we have done at page 258, and that the whole material universe has been formed from it and through it. This idea will assist physicists in forming their theory of a plenum of meteorites or meteoric matter, if such they choose to call it. It will also enable us to complete the circle of our notions with respect to matter. We believe that we can neither destroy nor produce the smallest portion of it, although we can change its form. Thus, looking upon the ether as primitive matter, we can understand how the solar system could be elaborated from it; and how, after having accomplished the purposes for which it was brought into existence, it may again be resolved into the primitive element out of which it was made, ready to take itspart in the evolution of some other system with, perhaps, a new earth "without form and void."
We have now to direct our thoughts, as far as we can, to the mass, which furnishes the really effective power of the sun as the ruler of the system; and, first of all, we have to think of what are the real active elements which form that mass. Hitherto we have looked upon them as all included within a diameter of 867,000 miles, but now we have to take notice of the clouds of meteoric matter which have been supposed by some astronomers and physicists to be revolving round the sun and continually raining into it; and of the enormous atmosphere which surrounds it. With regard to the former of these two elements, we shall compound our ignorance by looking upon it as a merchant does on his account of Bills Receivable, as not being available in the case of a sudden demand for cash, and therefore as not forming a part of the mass, any more than as the attraction of the earth aids the sun in its management of the planet Neptune; the same as the bills receivable strengthen the credit of the merchant. But with regard to the second element of the two, we must recognise that it forms part of the mass and power over the whole of the system, and from all that is known about it we are not authorised to look upon it as a negligible quantity. It so happens that the only thing we have to which we can compare it is the atmosphere of the earth, and we immediately find that there is absolutely nothing to be learnt from such a comparison. We know that one-half of the weight or mass of the earth's atmosphere is contained in a belt of 3½ miles high above its surface, so that double the volume of that belt estimated at atmospheric pressure gives us the true measure of its mass. This mass, when reduced to the density of water, and compared to that of the earth as we have dealt with it all along, turns out to be about 1/824,000th part of it; and were we now to add that to the earth's mass we have been using, its mean density would be 5·66065 instead 5·66 times that of water.
Now, let us suppose the sun to have an atmosphere of the same kind as the earth's: Seeing that the force of gravity atits surface is about 28 times greater than it is at the surface of the earth, a belt around it which would contain one-half of its mass would be 28 × 3½ = 98 miles, or say 100 miles thick. Dealing then with this dimension in the same manner as we have done in the case of the earth, we find that its supposed atmosphere would be 1/836,000th part of its mass, which, if added to the mass we have used for it, would make its mean density 1·413016 instead of 1·413 times that of water. Then again, if we suppose the earth's atmosphere to extend to 100 or 200 miles above its surface, the supposed atmosphere of the sun would extend to 2800 or 5600 miles above its surface, according to which of the above heights on the earth is adopted; whereas the highest of our authorities say that the corona, or apparent atmosphere, extends to at least 350,000 miles from its surface.
It would appear then that there is no analogy whatever between the atmospheres of the sun and the earth; but there must be some analogy, because the law of attraction cannot be suppressed at the surface of the sun; neither can any vaporous matter near it cease to be attracted in the same proportion as it is at the surface. Our atmosphere causes a pressure of 29½ inches of mercury at the earth's surface, and the attraction of the sun at its surface must cause a pressure equal to nearly 28 times that without fail, i.e. 420 lb. per square inch instead of the 15 lb. of the earth. We know that some spectroscopists believe that the pressure at the surface of the sun is sometimes as low as it is at the surface of the earth, even lower; but we require an explanation of why it is so. At the surface of the sun one second of arc corresponds to a height of 450 miles above its surface, and Mr. Proctor states in his "Sun," page 295, that if even "two or three hundred miles separated the lower limit of chromatosphere from the photosphere, no telescopes we possess could suffice (when supplied with suitable spectroscopic appliances) to reveal any trace of this space. A width of two hundred miles at the sun's distance subtends an arc of less than half a second; and telescopists, who know the difficulty of separating a double star whose components lie so close as this, willreadily understand that a corresponding arc upon the sun would be altogether unrecognisable." We can understand this, and perhaps find an explanation for ourselves.
According to our supposition that the sun may have an atmosphere similar to the earth's, at one hundred miles in height it would be reduced in pressure to 14 atmospheres, and, extending the analogy, at 2800 miles high the pressure would still be equal to one-eighth of 28 atmospheres, or equal to something less than 2 lb. per square inch at the surface of the earth; so that if spectroscopists have measured the sun's atmosphere at the disk, and found it to be lower than the earth's at its surface, their results must have been caused by some fortuitous circumstance which they did not notice at the time; because the force of attraction at the surface of the sun can never be overcome except by some counteracting force, which, if in the form of a vapour, or what we call a gas, issuing from its interior, would increase rather than diminish the pressure. We know that in the heart of a cyclone on the earth there is sometimes a vacuum sufficient to explode (pull out the walls of) houses near which it passes; and, at the same time, we know, more or less, what heat the sun sheds upon the outer atmosphere of the earth, and also the rate of rotation of the earth in the regions where the fiercest of these cyclones occur, the only two causes which can produce them. Now, if we compare these causes in the two bodies, that is, the earth's rotation of about 16 miles per minute and the sun's of, say, 60 to 75 miles per minute, and the temperatures of the sun and the earth at their respective surfaces, we can imagine that in the heart of a cyclone on the sun there may be a vacuum much nearer absolute zero than there can be in any one on the surface of the earth. If then the spectroscopists, without knowing it, have caught the spectra of the hearts of cyclones, we can conceive them to be right, otherwise no.
Again, we know that when big guns are fired off partial vacuums are formed near them, sufficient to cause disaster to windows, doors, and even walls of houses too near them, but whatever we may have said of force sufficient to produceexplosions in the sun, we have never believed that matter is ejected from the sun by explosions. We have supposed the sierra, or chromosphere, to have oozed out through its pores, sometimes to less, sometimes to greater heights, like steam from an open boiler, and the prominences to be eruptive, neither of which modes could produce anything approaching to vacua in their neighbourhoods. There can be no resemblance between the ejection of matter or gas from the sun and from a cannon, but there is between the ejection of vapours and the escape of steam from the safety-valve of a closed steam boiler; both of them continue to pour out their vapours till the pressure within falls down till it is equal to the resistance to their escape; there is no explosion, therefore no vacuum, appreciable at least, in the neighbourhood. There may be surrounding matter drawn up by the velocity of the outward current, but that is all.
Notwithstanding all this, we see no reason why the sun should not have an atmosphere of exactly the same kind as the earth's, composed of exactly the same kinds of gases, including vapour of water in some part of it, though, perhaps, far removed from the photosphere. Every other element found on the earth can be found in the sun, and so it is not unreasonable to suppose that the same kind of atmosphere may exist upon it; we have only to acknowledge that its conditions must be somewhat varied, all the difference being that the atmosphere of the sun must be heated up to the temperature of the photosphere where it comes in contact with it, while that of the earth is only of the temperature of the earth at its surface. In the case of the earth, if this were at a white heat, one-half of the weight of its atmosphere would not be comprehended in a belt around it of 3½ miles thick. That balance of mass might take place at a height of even hundreds of miles—we have no means of calculating how high—and still its pressure at the surface would be the same as now, as long as the earth's attraction remained the same; so must it be with the sun. Instead of limiting its height to 5600 miles at the utmost as we have done above, it would be no stretch of imagination to suppose that it might extend to ten, twenty,or more times that height. In addition to this we have to take into consideration that the sun's atmosphere must be swept up to something far beyond 5800 miles high by the whirlwinds created by the velocity of rotation at its surface, the same as we saw the earth's might be when we were explaining how an aurora could be made to glow at heights far beyond what we were accustomed to believe its atmosphere could reach. Adding, then, together these two motive forces for elevating the atmosphere of the sun, it would be a bold assertion to say that it cannot have one exactly similar to the earth's, reaching up to the height of 350,000 miles mentioned a few pages back. And now, having got this length, we may venture to assert that the corona of the sun is made up of this atmosphere, and of the vapours of the elements thrown out from its interior, somewhat in the manner we have described in last chapter; to which we have only to add that the bubbling up of vapours all around the sun, which produces the sierra or chromosphere, would not be interfered with in any way by the tremendous commotions which we have shown must be produced between the surfaces of the sun-spot zones and the centre; and that the projection of the high prominences would assist in elevating the aeriform atmosphere.
If then the sun has a compound atmosphere of this kind, it must be considerably more dense, proportionately, than that of the earth, and will consequently form a greater addition to its mass than we have found would be made by its airlike atmosphere. But, whatever density has to be added to it on that account has to be subtracted from the interior having been ejected from thence; because, in whatever manner its mass has been calculated in respect of the other members of the system, the total amount must turn out to be always the same. We have always estimated its mass from a diameter of 867,000 miles, which gave us a volume of 341,237,6389cubic miles, so that if we now include in the diameter the 350,000 miles height of the atmosphere, we get a volume of 2,053,50012cubic miles, which is as near as possible six times the volume in which we had to distribute the volume of the sun. How to do this, we know not. We cannotfix the region of greatest density in the same manner we have done atpage 221, but we know that it must be considerably nearer to the surface of the photosphere than we have there placed it; and of one thing we are sure, and that is, that the densities we have named for that region and the outer and inner surfaces of the shell, atpage 223, must be less than those there expressed; how much we cannot calculate, but we have certainly found that the limits must be lower, and that most probably there is no matter in the sun exceeding the half of the density of water.
Whatever the composition of the sun's atmosphere, or corona if that name be preferred, may be, spectroscopists have found in it aspectralline derived from some substance totally unknown to science. Now, looking back on our work from almost the very beginning, it seems to have been gradually borne in upon us that this unknown substance is the ether. That it is a material substance we were hardly ever in doubt, and our studies of it have substantiated and confirmed our belief. In our analysis of the Nebular Hypothesis inChapter VI., after combating the notion that the light of nebulæ is occasioned by incandescent gas, we showed, by the example of an air furnace, that an incandescent gas is composed of two elements, one consisting of solid matter which takes up and gives out heat and has all the properties of a heated solid or liquid substance, and the other of gaseous matter which, being the element that fills up the empty spaces between the solid atoms of a gas or vapour, only performs the office of carrying the solid part into the furnace. This forced upon us the idea of the gaseous part being a carrying agent, and very naturally to think of its being really the ether, that being the only acknowledged agent for the carriage of light, heat, and electricity, two of which are easily seen and felt, and the third cannot be awanting, in an air furnace. Again, when treating inChapter VII. of what effect the ether might have on the density of the original nebula, we concluded that its density must be much lower than what we then knew it had been estimated to be, and also that its temperature in space must be lower than -225°; which two circumstancescombined showed us that if it is a gaseous substance it must be very different to any gas that had been liquefied up to that time. This we repeated in great part inChapter XII., calling attention to the peculiarity of its being able to carry a higher temperature than its own—to all appearance—into a "hot box." Then we have dedicated two Chapters,XIII. andXIV., almost exclusively to the study of the ether, and have been led from one stage to another to look upon it as the only substance that agrees with the definition of a gas as given by science; true gas there is; as the primitive and sole element in the formation of all matter and in the evolution of the universe; and what is something more than an unfounded guess, as the mysterious and incomprehensible agent attraction, unfortunately almost universally spoken of as gravitation. And now to conclude: From what we have been able to learn, very slight differences have been found in various spectra of the position of the line representing the unknown substance, but this can cause very little doubt of its always being the same, as spectra often contain several lines of hydrogen, owing most probably to combinations with other substances; and if the ether is the primitive chemical element, there may be slight differences in the position of its line, as shown in all the phases in which we seem to have found it, but they must be slight as compared with the hydrogen lines, because even these must be in some measure, perhaps even great, influenced by the unfailing and inevitable mixture of the ether in their composition.
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[A]This temperature is altogether erroneous, as we shall show in due time; at present our proof would not be accepted without a demonstration, for which we have not sufficient data.
[A]This temperature is altogether erroneous, as we shall show in due time; at present our proof would not be accepted without a demonstration, for which we have not sufficient data.
[B]Here we beg to state that in all our coming operations, we will use the Centigrade Scale for temperatures without adding C to each number specified, unless a different scale has to be referred to, in which case the distinctive of the scale shall be given in the usual way. This we do because it is the fashion, not because we think it possesses any advantage over any other scale, but rather the contrary. Perhaps we may have something more to say about scales after we have handled the Centigrade a little more than it has been our lot to do hitherto.
[B]Here we beg to state that in all our coming operations, we will use the Centigrade Scale for temperatures without adding C to each number specified, unless a different scale has to be referred to, in which case the distinctive of the scale shall be given in the usual way. This we do because it is the fashion, not because we think it possesses any advantage over any other scale, but rather the contrary. Perhaps we may have something more to say about scales after we have handled the Centigrade a little more than it has been our lot to do hitherto.
[C]The exponent 18 in 150,523,772,69218means that 18 cyphers have to be added to complete the number. The same is the case with any other number and exponent of large quantities.
[C]The exponent 18 in 150,523,772,69218means that 18 cyphers have to be added to complete the number. The same is the case with any other number and exponent of large quantities.
[D]From the same source, date June 6, 1896, we learn that the greatest cold probably ever reached was -243·5° or 31·5° of so-called absolute temperature, but that will have very little effect on our calculations, and so it is not worth while altering them all to suit.
[D]From the same source, date June 6, 1896, we learn that the greatest cold probably ever reached was -243·5° or 31·5° of so-called absolute temperature, but that will have very little effect on our calculations, and so it is not worth while altering them all to suit.
[E]Years after this was written we have seen it stated that the density of the ether has been calculated from the energy with which light from the sun strikes the earth, and that to represent it there are twenty-seven cyphers after the decimal point before the figures begin. But as this gives something like one thousand quadrillionth part of the density of water, we refuse to accept or even think of it.
[E]Years after this was written we have seen it stated that the density of the ether has been calculated from the energy with which light from the sun strikes the earth, and that to represent it there are twenty-seven cyphers after the decimal point before the figures begin. But as this gives something like one thousand quadrillionth part of the density of water, we refuse to accept or even think of it.
Transcriber Notes:Obvious misspellings and omissions were corrected.The illustrations have been moved so that they do not break up paragraphs and so that they are next to the text they illustrate.Some tables were reformatted.Errors in punctuation and inconsistent hyphenation were not corrected unless otherwise noted.
Transcriber Notes:
Obvious misspellings and omissions were corrected.
The illustrations have been moved so that they do not break up paragraphs and so that they are next to the text they illustrate.
Some tables were reformatted.
Errors in punctuation and inconsistent hyphenation were not corrected unless otherwise noted.