which is the more probable; if our notions of the cometary state be just, we cannot deny that the two first must produce some effect: but the greater part of the phenomenon may still be due to the third or some other cause.

The only remaining point of any magnitude, connected with the known facts of comets, is the question, whether there is any fluid medium in space, of such specific gravity, as to offer a sensible resistance to their motion. The question of resistance, or no resistance, is one of great importance, as the stability of our system depends in a great measure upon it. The solar system would be said to be stable, strictly speaking, if it were so constructed that its motions might continue for ever in the manner now observed, without any such change, arising from the mutual attractions, as would endanger the safety of any one planet. If a number of planets were launched into space, without any particular arrangement of their motions, the chances for the stability of such a system would be very small. We observe in the solar system the following facts, which mathematical analysis shows us are all necessary to its stability, as far as the planets are concerned :— 1. The sun is very much greater than any one of them, and the planets are placed at such distances from one another, that the attraction of the sun upon each is always very much greater than that of the other planets. 2. They all move in the same direction round the sun. 3. The orbits are very nearly circular, and are inclined to one another at small angles. Froin these circumstances, and the law of gravitation, it has been deduced that the average distances of the planets from the sun, and also the average motions, are invariable, or at least will continue the same for a number of ages, which, to our limited ideas, give the notion of eternity. But all this is on the supposition, that there is no fluid which offers any resistance to the planetary motions; if such a fluid exist, however small its density may be, it can be shown that it continually decreases the mean distances, and increases the mean motions. Observation teaches us, that the mean distances of the planets are invariable, or at least that, if they do change at all, the variation is so small, that it has not become perceptible to our best instruments in hundreds of years. The resisting fluid, therefore, if it exists, is of an extremely small density. Comets offer the only chance left to us at present of settling the question; being of very little density themselves, their motion will encounter more resistance than that of the planets, for the same reason that a feather will fall more slowly to the ground than the same weight of iron. If there be a resisting medium, it will certainly diminish the mean distance of the comet, and increase its mean motion; and this phenomenon has been observed in the comet of Encke, which is the one with which we are the best acquainted. Professor Encke says *

*In a dissertation which appeared in the Astronomische Nachrichten, and has been

--

"If I may be permitted to express my opinion on a subject which for twelve years has incessantly occupied me, in treating which I have avoided no method, however circuitous, no kind of verification, in order to reach the truth as far as it lay in my power; I cannot consider it otherwise than completely established, that an extraordinary correction is necessary for Pons' comet - that is, the one which all the world, except Professor Encke, calls Encke's comet "and equally certain that the principal part of it consists in an increase of the mean motion proportionate to the time." Professor Airy adds, "I cannot but express my belief, that the principal point of the theory, namely, an effect exactly similar to that which a resisting medium would produce, is perfectly established by the reasoning in Encke's memoir." If this conclusion be correct, we may predict that, in time, this comet and every other will fall into the sun; we know, however, that the medium, if it exists, cannot sensibly affect the planetary motions for a great number of centuries.

There seems to be some cause in operation by which the brilliancy of comets is continually diminishing. That of Halley, in one of its preceding revolutions, is described as giving a degree of light certainly superior to that which it gave in 1682 and 1759. Sir John Herschel could only see Biela's comet through a reflecting telescope of twenty feet in length, an instrument of enormous power in the collection of light; and though he afterwards found it with a refracting telescope, he asserts that he never should have succeeded with the latter, unless he had previously known where to look for it. If the parts of the comet have so little cohesion, as has been, with great probability, conjectured, it may easily lose a part of its substance as it passes through a resisting medium. We have however as yet but little specific information on this subject.

With regard to the cause of the tails of comets, we can say nothing with certainty. Their existence affords a strong presumption for the very little density of the nuclei. They were at one time considered as being in a continuation of the line drawn from the sun to the comet; it has, however, been shown, that they always fall a little behind this line with respect to the comet's path, and have sometimes been even perpendicular to it. That of 1680 was 90° of the heavens in length, so that part of it might have been in the observer's zenith when the comet was setting. It was 141 millions of miles in length. Some comets have had what we may call a succession of tails, one succeding another,

translated into English by Professor Airy, with an Appendix in which the latter gentleman fully coincides in the conclusion of Professor Encke. Those who are acquainted with the present state of science will give great weight to these authorities, to say nothing of their calculations being before the world,

with a vacant space between every two. The conjectures as to the nature and formation of these singular attendants are entitled to very little attention.

As to the multitude of idle theories with which, for want of better information, this part of astronomy has been loaded, such as that the planetary system was formed by matter struck off from the sun by one comet; that another caused the deluge; that the four small planets were formerly one, which was broken in pieces by a third; that the moon was originally a comet, and the like; - we would willingly amuse our readers by an account of them, if our limits permitted. They will however find them all, handsomely exposed by M. Arago, in the Annuaire already cited. If any, or all of them, should be hereafter proved to be true, it will be no excuse for those who first made them; for a result produced on insufficient evidence is bad, whether true or false. As the science of astronomy approaches towards perfection, we shall doubtless add some important and interesting facts to our knowl edge of comets.

Elements of the Orbits of the three Comets, which have appeared aocording to prediction, taken from the work of Professor Littrow, "Ueber den gefurchteten Cometen des gegenwärtigen Jahres 1832, &c. Vienna, 1832."

The comets of Encke and Biela move according to the order of the

signs of the zodiac, or have their motions direct; the motion of that of Halley is retrograde.

II. — INFLUENCE OF THE MOON.

In the "Annuaire pour l'An 1833, présenté au Roi par le Bureau des Longitudes," there is an interesting paper furnished by M. Arago of Paris on this question, viz. "Does the Moon exercise upon our atmosphere any influence that can be estimated?" M. Arago begins his essay with the following remarks.

"Astronomers, natural philosophers, and meteorologists seem generally convinced that the Moon does not exercise upon our atmosphere any influence that can be estimated; but it must be confessed they only are of this opinion. The immense majority of mankind firmly believe in a powerful action of our satellite. Agriculturists, and especially seamen, say that they have remarked, in a thousand instances, that every change from one phasis of the moon to another, leads to a change of weather. “A question so complicated cannot now be resolved by merely theoretical considerations. It is only by long courses of accurate observations, methodically arranged, that we can hope to obtain results that cannot be controverted, or that are worthy of a place in the science of meteorology. Unfortunately labors of this kind are very few in number, and embrace only short intervals. The inquiry, however, having made some indisputable progress, it has been thought that it was time that notice should be taken of it. For my part I should have preferred to wait for the accomplishment of a numerical discussion of observations at Paris, in which M. Bouvard is actually engaged. Further, if this first attack upon prejudices deeply rooted, is without effect, I shall ask of the readers of the Annuaire permission to recommence the combat, when meteorology shall have gained the new acquisitions, which every thing authorizes us to expect."

As we have not room to insert the whole of M. Arago's article, we extract from the "Foreign Quarterly Review" for April, 1833, a brief abstract of that portion of it which relates to the influence of the moon on rain and on the changes of the weather.

"The first question which M. Arago undertakes to examine is, whether the moon exercises any influence on the rain. The data on which he founds his remarks are derived from a series of observations published by M. Schübler, a professor at Tubingen. They comprehend a period of twenty-eight years, and were made at the following places, namely, at Munich, from 1781 to 1788; at Stuttgard, from 1809 to 1812; and at Augsburg, from 1813 to 1828.

"From the comparison of these observations it appears that the maximum number of rainy days takes place between the first quarter and the full moon, and the minimum, between the last quarter and the new

moon. The number of rainy days in the last of these intervals, is to that in the first, as 696 to 845, or in round numbers, as 5 to 6. And this proportion is not only true of the twenty years taken together, but also of the separate groups of four years, which give analogous rumbers. We are therefore to conclude, if we put faith in the observations, that it rains more frequently during the increase, than during the wane of the

moon.

"The above results are confirmed by a series of observations made at Vienna, and discussed by Pilgram in the year 1788. On 100 repetitions of the same phasis, Pilgram found the falls of rain to be as follows: new moon 26, mean of the two quarters 25, full moon 29; consequently, at Vienna, as well as at Augsburg and Stuttgard, it rains more frequently on the day of the full than on that of the new moon.

"Another element remains to be taken into consideration, namely, the moon's distance from the earth, which, admitting the lunar action on the atmosphere, it is natural to suppose will have a marked influence on the phenomena. In fact, Schübler found that during the 371 anomalistic revolutions of the moon which take place in twenty-eight years, the number of rainy days included within the seven days nearest the perigee, was 1169, and within the seven days nearest the apogee, 1096. From the observations at Vienna, Pilgram found that during 100 lunations the number of rainy days at the perigee was 36; and at the apogee, 20 only. Thus, other circumstances being alike, the nearer the moon is to the earth, the greater are the chances of rain.

"Confining ourselves,' says Arago, ' to the principal results, it seems difficult to resist the conclusion that the moon exercises an influence on our atmosphere; that in virtue of this influence rain falls more frequently towards the second octant than at any other epoch of the lunar month; and lastly, that the chances of rain are fewest between the last quarter and the fourth octant.'

"The influence of the moon on the terrestrial atmosphere seems also to be rendered evident by observations of a different kind, namely, the mean heights of the barometer at the different lunar phases. On calculating a series of observations made at Padua by the Marquis Poleni, and extending over a period of 45 years, Toaldo found that the mean height of the barometer at the quarters is greater than its mean height at the syzygies, and that the difference amounts to 0.46 millimetres.

"From the observations of M. Flaugergues, made at Viviers in the department of Ardèche, and comprising a period of 20 years, there results: mean height at the quarters 755.81 millimetres; mean height at the syzygies 755.39; difference 0.42.

"From a series of observations made in the Royal Observatory at Paris, and discussed by Bouvard, the following results were found: mean height at the quarters 756.53 millimetres; mean height at the syzygies, 755.90; difference, 0.69.