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have already mentioned the comet of 1770; but this, it may be said by those who cannot examine the calculations for themselves, was a trick of the astronomers, to account for their own failure. We will therefore cite another instance, in which the effects of planetary perturbation were very great, were predicted before the event, and verified by it. The comet of 1682, or of Halley, it is well known, was predicted by him as likely to appear in 1757. This he concluded from observing that a comet with a similar orbit had appeared in 1531 and 1607. He however remarked, that as the comet would, if his supposition were true, pass near to Jupiter and Saturn, some alteration might be expected from the attractions of these planets. In 1757, while astronomers were beginning to look for the expected body, with no very great hopes of its reappearance, Lalande proposed to Clairaut to undertake the computation of the effect of the planets upon the comet. These names may not be so well known to our readers as to mathematicians and astronomers; we will, therefore, inform them, that Lalande was a practical astronomer of great eminence, and that Clairaut was a mathematician and natural philosopher of even greater celebrity. So little wedded were these men to the system of gravitation, that the first discarded, or at least, threw doubt upon, the theory of the return of comets, on account of the non-appearance of that of 1770, already mentioned; while the second, on account of some unexplained phenomena, imagined that Newton had mistaken the law according to which the mutual attractions of planets depend upon their distance. The two undertook the enormous labor above-mentioned; and the result was, that Clairaut announced, in the year 1758, that the revolution which was actually taking place, would be 618 days longer than the preceding one, that is, the one which took place between 1607 and 1682. At the same time, he observed, that the methods of calculation were yet so incomplete, that the result could not be depended upon within thirty days. If his conclusion had been quite correct, the comet would have come to its perihelion, or nearest point to the sun, about the middle of April, 1759; and it did arrive there on the 13th of March of the same year, within the thirty days which had been allowed for errors. We may further remark, that the comets of 1832, of which that of Encke has once before appeared, according to prediction, and that of Biela* has been already observed by Sir J. Herschel, both very near their predicted places, could not have had their tables constructed without a strict attention

* On the subject of this comet, we derive our information from a communication made by Sir John Herschel to the Royal Astronomical Society, and read at the meeting of that body on the 9th of November last. An account of this interesting paper may be found in the Monthly Notice of the transactions of the Society for November, published by Priestley and Weale, Holborn.

to the planetary perturbations. assuming that comets are material bodies, subject, like the planets, to the attraction of the sun and other bodies of our system, and describing an elliptic orbit round the sun nearly, the difference being attributable to the action of the planets, or, perhaps, in some degree, to a resisting medium.

From such facts we are justified in

The next question is, comets being material, what is their quantity of matter: that is, if brought to the earth without alteration of their dimensions, would they be light or heavy in proportion to their size. On this point we have sufficient evidence, not as to the actual quantity of matter in any comet, but as to limits below which it must fall, at least in all the comets of which the times of revolution are known. It results from the theory of gravitation, that of two bodies, the first cannot affect the second, without being itself more or less affected by the second. And of two bodies, one of which is very great compared with the other, the effect which the smaller produces upon the greater is small compared with that which the greater produces upon the less. This is analogous (though the two phenomena must not be confounded) to a fact of every day observation, that a light body striking against a heavy one, though with with great velocity, produces, nevertheless, but a small change in the velocity of the greater one, and vice versa. For example, in the motion of Jupiter and Saturn it is observed, that the average velocity of Jupiter is accelerated, while that of Saturn is retarded more than twice as much. And it is shown, by a process independent of this observation, that Jupiter contains more than twice the quantity of matter of Saturn. After some ages, the motion of Jupiter will cease to be accelerated, and that of Saturn to be retarded. After which, that of Jupiter will begin to be retarded, while that of Saturn will begin to be accelerated. Hence, if a comet so large, or rather so heavy, as to bear an appreciable proportion to the mass of a planet, were to be disturbed by the latter in any considerable degree, the comet itself would produce a degree of disturbance in the motion of the planet, which would be perceptible to our instruments. Thus, if Halley's comet, which was retarded between 1682 and 1579, more than 500 days by the action of Jupiter, had been only the twenty-thousandth part of the mass of Jupiter, its effect upon the latter would have been even then most distinctly perceptible by good instruments. The same thing would take place now if the mass of that comet were very much less, and yet, in the former case, it would be less than one sixtieth part of the earth. But there are two much more conclusive arguments. Laplace found, that if the comet of 1770 had only been the five-thousandth part of the earth, it would have lengthened our year by three seconds. No such alteration has taken place, and the comet must, therefore, have been less than the five-thousandth part of the earth. The same body passed between G*

the satellites of Jupiter in 1779 without producing any effect; a very little quantity of matter, much less than the five-thousandth part of the earth, would have been sufficient to derange that system perceptibly.

But, it may be asked, are we certain that we know the length of the year with such accuracy, that a difference of three seconds would be of "sufficient magnitude to be discoverable by our instruments? To give an idea of the possibility of this, we will state the following fact. Some years ago, Professor Airy of Cambridge, proposed a method of determining the moon's mass, which required accurate observations of Venus near her conjunction. An ephemeris of this planet was accordingly prepared, containing the computed time at which the planet should pass the meridian daily, for that part of the year 1830, in which the conjunction of Venus happened; this was forwarded to different astronomers, English and continental, with a request that they would observe the real time of the meridian passage at their various observatories. Among the observations which were made in consequence, those of Professor Santini, of Padua, were so arranged as to show how much they differed from the ephemeris. The difference was, in only a very few instances, so great as one second, and was, for the most part, nearer to half a second. And this result is not considered as anything remarkable.

The appearances of comets are, as far as appearances can be, proofs of their very small mass. The phenomenon of their tails, adopt what explanation we may, can only be accounted for on the supposition that the comets themselves are of very small density. But even the nebulous head of the comet has often been so rare, that small stars, which a fog of moderate intensity would hide, have been seen through its most central parts. Thus Seneca mentions the fact of stars having been seen through comets; Sir W. Herschel saw a star of the sixth magnitude through the centre of the comet of 1795; Professor Struve saw one of the eleventh through that of Encke; and Sir John Herschel, in the Memoir already cited, (in col. 2, note) informs us, that on the evening of the 23d of September last, he saw a whole cluster of stars of the sixteenth magnitude, almost through the very centre of Biela's comet, the light of which, according to Sir J. Herschel, could not have passed through less than 50,000 miles of the matter of the comet. As neither of the gentlemen above quoted saw any effects of refraction which would have been very apparent had the cometic matter been even many times rarer than our atmosphere, (if, indeed, they could have been seen at all through such a mass, which may fairly be doubted,) we are entitled to conclude that those comets, at least, which are best known to us, are of a rarity far exceeding that of any matter as it exists at the surface of our globe. 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 positively denied the fact would deserve the same censure.

As we are not writing for the scientific part of the community, we will say a few words on a very general fear which prevails - namely, that the near approach of a comet would break our planet in pieces, or at least produce a great accession of heat, sufficient perhaps to destroy animal and vegetable life, if not to burn the world altogether. The argument seems to have originated in a notion, that because heat produces expansion, therefore very highly expanded bodies must needs be very hot. It would be as good an argument to say, that because expansion by any other means except heat, produces cold, that therefore all comets must be very cold; and neither argument would, in the least degree, afford matter even for a rational conjecture. We can form so little idea of what the state of a planet of vapor, it may be consisting only of one sort of matter, would be, that we might with as much reason speculate upon the possible organization of the possible animalcula which swim in that vapor, as try, in the present state of our knowledge, to ascertain whether any and what degree of danger awaits us from such a source. A comet may certainly strike the earth in.the next century; not one of these which are known, unless the laws of nature be singularly altered, but some one or other yet to come. It has been shown, but by considerations of so high a nature that the result cannot be expected to bring much conviction to any but a mathematician, that if a comet were launched at hazard into our system, for one orbit in which it could strike the earth there are 281 millions in which no such thing could take place as the laws of nature stand at present. The advocates of cometary interference (we have met with some whose manner of expressing their opinion on the subject almost entitles them to that name) usually suppose a special interposition of the Divine power, which, (resting on their own interpretation of certain Scriptural prophecies,) they suppose will bring a comet on the earth. They are usually people of some religious feeling, and would act more consistently with the idea they ought to have of their own ignorance and the Divine power, if they ceased to prescribe to the Creator in what way it should please him to alter the course of events which it has hitherto been his will to arrange. It is impossible to produce any other argument on the subject, consistently with the design of this paper; the province of natural philosophy is to collect and compare facts, and to say what will be, if things continue as they have been; it never presumes even to conjecture what shall be, when the power which has hitherto disposed events in one manner, shall judge it right to ordain a different arrangement.

There are many who, without going the length of fearing danger from the shock of a comet, nevertheless imagine that any, unusually hot weather which happens while such a body is visible, or going to be visible, is caused by it in some measure at least. To such a circumstance the fine vintage of 1811 was attributed, and many, even among the edu cated classes, imagined that the heats of last September and August were occasioned by the approach of Biela's comet. We can certainly reecho, from this side of the channel, the complaint which M. Arago makes, in the Annuaire for 1832, already alluded to, of the scarcity of the meanest knowledge of scientific facts among the middle ranks of society. With a burning sun over head, we have heard those, who might have known better, accusing the comet in the manner aforesaid. It appears, however, from the table of M. Arago, in which the mean temperature of every year, from 1803 to 1831 inclusive, is placed side by side with the number of comets observed in that year, that there is no visible connexion between the one and the other. Thus 1806 and 1811 were both hot years, the first however hotter than the second, though the first had one comet only of no note, and the second had two, one of which was the most brilliant which the present generation has seen. Again, the year 1826, with its five comets, was not so hot as 1831, which had only one. That hot years in general have more comets than cold ones is very true, and for this simple reason, that the former, generally giving a finer sky, are more favorable for their discovery. We must not forget that the greater number of such bodies are not visible to the naked eye. Thus all the years between 1803 and 1831 inclusive, the temperature of which exceeded the average, mustered twenty-nine comets between them; and the remaining, or cold years, only fifteen. We must therefore say, not that the comets brought the heat, but rather that the heat brought the weather which made the comets visible. In the period above-mentioned there were forty-four comets observed, counting distinct appearances of the same comet as different; of which only two were in the least remarkable for brilliancy those of 1811 and 1823.

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Having shown that some comets are bodies in the highest state of tenuity, and conjecturing, with a great degree of probability, that the same is true of all, we may mention a phenomenon which has been several times remarked by different observers, viz., that in their approach to the sun they appear to contract their dimensions, or the nebulous head of the body diminishes in apparent diameter. As they recede from the sun they begin to dilate again. To explain this phenomenon, some have had recourse to the highly elastic fluid or ether, which, as we shall presently see, has been supposed to fill the solar system at least. If this ether, say they, be denser as we approach nearer the sun, we must expect that the comet will be more compressed by it as it ap

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