from the sun, its distance is about four hundred and forty-three millions of miles, which is nearly four and a half times the earth's distance, and is little less than the distance of Jupiter. The orbit is inclined to that of the earth at nearly thirteen degrees. This comet may be considered as a planet, revolving within the orbit of Jupiter, and nearly in the common plane of the solar system. Its motion also, as well as that of Biela's, is in the same direction as that of the planets.

In the calculations of Encké for the determination of the movement of this comet, the most scrupulous account was taken of the effects which the planets must produce upon it. Nevertheless, a small discrepancy was found to exist between its observed and computed returns in 1822, 1825, 1829, and 1832; and what was still more remarkable, this discrepancy was of the same nature in every case; so that it is impossible to suppose that it could have arisen from any casual error of computation or of observation; since, had it so occurred, it would have affected the result irregularly. We must therefore conclude, that this comet does not precisely retrace its steps each revolution. It is found, however, that this irregularity, from whatever cause it may proceed, does not disturb the plane of the comet's path. It is in fact, according to the observations and reasonings of Professor Encké, precisely the effect which would be produced if the space through which the comet moves was filled by a subtle fluid, offering a small resistance to the motion of the comet; just as our atmosphere resists the motion of any light body through it.

The existence of an extremely subtle etherial fluid which fills the infinitude of space has been adopted hypothetically to explain the phenomena of optics. In fact, light itself is, according to the undulatory theory, supposed to consist in vibrations transmitted through such a fluid, just as sound is known to consist in similar undulations transmitted through the atmosphere. Hitherto this assumed cause for light has been justly regarded as an ingenious hypothesis not proved, but which accounts for the various phenomena more fully and satisfactorily than the corpuscular theory; which, besides being open to the same objection, completely fails when applied to some phenomena of light, which recent investigations have developed. If an effect similar to that which has been observed in Encke's comet should be discovered on the approaching return of Halley's comet, and still more, if it be observed on the next return of Biela's comet,*

*The last return of this comet anticipated its calculated return by one day-Encke's comet loses two days of its period every revolution.

the undulatory hypothesis will begin to assume the character of a vera causa; and that theory of light must, under such circumstances, be considered as established.

The effect on the return of a comet produced by this resistance, contrary to what might at first be expected, is to accelerate it; or to make the actual return anticipate the return as computed on the supposition that the comet moves in an unresisting medium. This difficulty will, however, be removed, if it be remembered that a resisting medium, by diminishing the velocity of the body in its orbit, diminishes the influence of the centrifugal force to resist solar attraction. The body, therefore, follows a path constantly nearer to the sun;-in other words, the orbit is in a progressive state of diminution. Now, the less the orbit is, the less time necessary to describe it, and consequently the shorter the period of the successive returns of the body to the same position.

The return of Halley's comet having been computed without taking into account the effect of this supposed resistance, it is possible that its actual may anticipate its predicted arrival. From the small effect, however, produced upon the successive returns of Encke's comet, it is not probable that the event will, on this account vary very materially from the prediction.

If the successive returns of the periodic comets should establish satisfactorily the existence of the luminous ether, it will follow that after the lapse of a certain time every comet will ultimately fall into the sun. In every succeeding revolution of the same comet, its path would fall a little within its former course, and it would describe a spiral line round the sun, continually approaching that body, until at length it would arrive close to his surface: before this would happen, it would doubtless be wholly converted into a light gas by his heat, which would probably mingle with the solar atmosphere.

In the efforts by which the human mind labours after truth, it is curious to observe how often that desired object is stumbled upon by accident, or arrived at by reasoning which is false. One of Newton's conjectures respecting comets was, that they are the aliment by which suns are sustained;' and he therefore concluded, that these bodies were in a state of progressive decline upon the suns, round which they respectively swept; and that into these suns they from time to time fell. This opinion appears to have been cherished by Newton to the latest hours of his life he not only consigned it to his immortal writings, but at the age of eighty-three, a conversation took place between him and his nephew on this subject, which has come down to us. 'I cannot say,' said Newton, when the comet of 1680 will fall

into the sun; possibly after five or six revolutions; but whenever that time shall arrive, the heat of the sun will be raised by it to such a point, that our globe will be burned, and all the animals upon it will perish. The new stars observed by Hipparchus, Tycho, and Kepler, must have proceeded from 'such a cause, for it is impossible otherwise to explain their sud'den splendour.' His nephew upon this asked him, Why, when he stated in his writings that comets would fall into the sun, did he not also state those vast fires which they must produce, 6 as he supposed they had done in the stars? Because,' replied the old man, the conflagrations of the sun concern us a little more directly. I have said, however,' added he, smiling, ' enough to enable the world to collect my opinion.'

6

It may be asked, if the existence of a resisting medium be admitted, whether the same ultimate fate must not await the planets? To this enquiry it may be answered, that within the limits of past astronomical record, the etherial medium, if it exist, has had no sensible effect on the motion of any planet. That it might have a perceptible effect upon comets, and yet not upon planets, will not be surprising, if the extreme lightness of comets compared with their bulk be considered. The effect in the two cases may be compared to that of the atmosphere upon a piece of swan's down and upon a leaden bullet moving through it. It is certain that whatever may be the nature of this resisting medium, it will not, for many hundred years to come, produce the slightest perceptible effect upon the motions of the planets.

Biela's comet moves in an orbit whose plane is nearly the same with those of the planets. It is but slightly oval, the length being to the breadth in the proportion of about three to two. When nearest to the sun, its distance is nearly equal to that of the earth; and when most remote from the sun, its distance somewhat exceeds that of Jupiter. Thus it ranges through the solar system, between the orbits of Jupiter and the Earth.

Excepting these three comets of Halley, Encké, and Biela, there is no other whose periodicity has been satisfactorily established; that is to say, a prediction of whose return has been fulfilled. Dr Olbers observed a comet in 1815, whose return he has predicted in 1887.

The great comet of 1680 was conjectured to be identical with comets which had appeared in 1106, in 531, and in 43, B.C., the intervals being 575 years. This conjecture, however, rests altogether upon the equality of the intervals of its appearance; the path not having been observed antecedent to 1680. Should the

conjecture be well founded, it cannot be reifed mai avert the year 2255.*

Notwithstanding the discovery of the periodie somers of Latte and Biela, still the comet of Haley maintains a paramOMA 2007nomical interest; and may be considered to sant Lione ir exhibiting those physical phenomena which seem to be the en elusive characteristics of the class to which it belongs. Anot the comets of Encké and Biela are cogestionat y sopeers if interest to the geometer and astronomer, yet their shom pemode, the limited space within which they are cremanet in ther motion, the small obliquity and eccentricity of their ortute, and consequently the very slight disturbance will they still from the attraction of the planets, render them for a physics prompts nothing more than new planets of inappreciate muss belonging to our system. Unlike other known ermets, they do not rush from the invisible and inaccessible depths of space, and, after sweeping our system, depart to cistances under the exoseption of which the imagination itself is confounded. They possess none of that grandeur which is connected with whatever appears t break through the fixed order of the universe. It is still reserve for the comet of Halley alone to exhibit a phenomenon, so far as we know, unique-to afford a splendid result of those powers of calculation by which we are enabled to follow it through the depths of space, two thousand million of miles beyond the extreme verge of the solar system; and, notwithstanding disturb ances which render each succeeding period of its return diferent from the last, to foretell that return with precision.

By far the greater number of comets appear to be mere masses of vapour, totally divested of all concrete or solid matter. So prevalent is this character, that some observers hold it to be universal. Seneca mentions the fact of stars having been distinctly seen through comets. A star of the sixth magnitude was seen through the centre of the head of the comet of 1795 by Sir William Herschel; and in September, 1832, Sir John Herschel, when observing Biela's comet, saw that body pass directly between his eye and a small cluster or knot of minute telescopic stars of the sixteenth or seventeenth magnitude. This little constellation occupied a space in the heavens, the breadth of which was not

This is the comet to whose near approach to the Earth Whiston attributed the Deluge; the interval of time between 1680 and the period assigned to the Deluge, either by the Hebrew or Septuagerian chronology, being very nearly an exact multiple of the supposed period of the

comet.

VOL. LXI. NO. CXXIII.

H

the twentieth part of the breadth of the moon; yet the whole of the cluster was distinctly visible through the comet. 'A more 'striking proof,' says Sir John Herschel, 'could not have been offered of the extreme translucency of the matter of which this 'comet consists. The most trifling fog would have entirely 'effaced this group of stars, yet they continued visible through a thickness of the cometic matter, which, calculating on its dis⚫tance and apparent diameter, must have exceeded fifty thousand miles, at least, towards its central parts.' It is plain, therefore, that in this case, whatever may be the nature of this substance, it possesses no perceptible power either of absorbing or refracting the light which passes through it; and therefore, according to all probability, is of a density bearing a proportion which, in popular language, may be said to be infinitely small compared with the density of atmospheric air. If any man should assert that the 'largest comet ever seen, including its millions of miles of tail, 'contained no more matter than is to be found in the New River 'Head, he might justly be blamed for asserting more than he ' knew. But certainly any one who would positively deny the 'fact, would deserve the same censure.'*

Nevertheless, M. Arago leans to the opinion, that some of the comets which have appeared have a solid nucleus within the nebulous matter which surrounds them. This opinion he grounds upon the intense splendour which has been imputed to several of the recorded comets-such, for example, as in that which appeared in the year 43, B.C., and which the Romans considered to represent the metamorphosis of the soul of Cæsar. This was said to be visible in the presence of the sun. In 1402, another comet appeared, so brilliant, that the light of the sun, in the month of March, not only did not prevent the nucleus, but even the tail from being seen. We should attach to this example greater importance, but for the latter part of the statement. ever doubt there may be respecting the solidity of the matter forming the nucleus of some comets, there can be none respecting the tail; and it does appear to us something little less than incredible that the tail of any comet could have been seen in the presence of the sun. On the whole, however, M. Arago's inference is, that while there are many comets without any nucleus, there are some with a nucleus which perhaps may be transparent; and others more brilliant than planets, having a nucleus which is probably solid and opaque. The comets which are most intimately connected with our system-those of Encké and Biela

* Mr De Morgan, in the articles already cited.

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