to speak, in a few seconds, from the distance of Venus to that of the Earth, of Mars, Ceres, and Jupiter? If this, then, be true, and seeing the comet with our telescope supplied with its weakest magnifying powers when it crosses the orbit of Venus, we subsequently examine it in succession with the help of a magnifying power 3 times, 8, 17, 28 times stronger; then, if it be always seen, it ought to appear the same as when seen with the first and feeblest magnifying power, at the epochs in which its proper movement shall have transported it to distances from the sun, equal to the radii of the orbits of the Earth, of Mars, Ceres, and Jupiter; and if it be not seen, for example, when it has reached the orbit of Jupiter, it is not only because it has been subjected to the enfeebling which might result from the scattering of the matter of which it is formed;-it is because it does not act as a body which has an inherent light; it is because it borrows its brightness from the sun!

It is true that all comets are not equally eligible for these kinds of experiments. We ought to select in preference those comets without apparent nucleus, and without a tail, because they appear to be less subject than others to sudden and irregular changes of figure, and because that, during the act of the singular dilatation they undergo in removing from the sun, and of which Mr Valz has given the law, it is probable that all the parts, from the centre to the circumference, undergo analogous changes. Without this condition, the natural dilatation of the nebulosity cannot be assimilated to that which we obtain artificially in the foregoing proof by means of the eye-glass of the telescope. The importance of this remark will be perceived, when I notice that, in the comet of 1770, the nucleus and the nebulosity properly so called, were very far indeed from experiencing proportional changes *.

* In proof of this assertion, I subjoin the measurements which Messier has given both for the nucleus and nebulosity of the comet 1770 :On the 17th June 1770, the nucleus measured 0.22, the nebulosity 5.23

18.0

"

22d

0.33

...

...

The method which I have now described so much at length, is exposed, I believe, to only one kind of objection; it may be imagined that the matter of which the comet is composed has no inherent light of its own, but that it acquires it when under the action of the solar rays.

This hypothesis, in reality, would be nothing more than the reproduction of that system which Euler has developed in his "Letters to a German Princess ;" and, according to which, the light which enables us to perceive certain bodies, such as paper, porcelain, &c. is not composed of rays which are truly reflected, but rather of a particular kind of light, which these bodies engender by being subjected to vibrations under the action of the solar rays. This, it will be perceived, is a difficulty of pure theory, and which might be as much applied to the light of the moon, of the planets and their satellites, as to that of comets. But the sole object which I proposed to myself in this paper, was to discover a method which would enable us to decide whether these bodies were to be ranged, as it regarded their luminous properties, in the same category as our own globe, or as Mars and Jupiter, &c. That other question, whether the light which enables us to perceive coloured bodies is reflected, as Newton supposed, at the surface of very fine material plates, or proceeds from vibrations communicated to the air by the constituent particles of bodies; this question, I say, is of a very different kind and extent, and this is not now the place to discuss it.

3. On the Comet which will pass its Perihelion in November 1835.

In the Annuaire of 1832 I published a long notice, in which there is an elementary explanation of all that astronomy at the present day possesses of precise and mathematical information on the motion and nature of comets. To that treatise I beg to refer the reader. In the present note I shall content myself with mentioning some modifications made on the former results, pointing out the principal circumstances which may render this sixth appearance of the same comet useful to science, and indicating the constellations in which we are to seek it.

After having determined, by the assistance of very laborious

calculations, the derangements or perturbations which the comet at present expected, and known by the name of Halley's comet, must encounter during its progress, from the united attractions of Jupiter, Saturn, Uranus, and the Earth, Messrs Damoiseau and De Pontécoulant fixed, the one the 4th, the other the 7th November, for the time of the comet passing its perihelion, that is, the point of its orbit which is nearest the sun. Since these first researches, astronomers have determined, that the mass of Jupiter, which had been considered as equal to the 1070th part of that of the sun, is equal to the 1054th part. By adapting this new calculation, and by taking more completely into account the action of the earth, M. de Pontécoulant has definitely fixed the passage of the perihelion for the 13th instead of the 7th November.

At the moment of this passage, the distance of the comet from the sun will not exceed six-tenths of the distance of the sun from the earth. At the other extremity of the great axis of the orbit, in thirty-nine years from this time, the distance of the two heavenly bodies will on the contrary be immense. Calculation gives more than thirty-five times the radius of the terrestrial orbit, that is, more than thirty-five times the distance of the earth from the sun.

The result of the calculation of the passage of its perihelion by Halley's comet in 1835, compared with the result of observation, will inform us if this comet, like the small and feeble one which returns at short intervals, is sensibly deranged during its progress by the resistance of the ether. This comparison, in its turn, will furnish us with some notions on the physical constitution of the comet which we expect, for a given resisting medium exercises more or less effect according to the volume and density of the body which traverses it.

Is the ether in repose, or does it move round the sun, from west to east, in the manner of the planets? In the last case, its effect on the comet, which returns at short intervals, which itself moves from west to east, will be different from that which it will exercise on the comet of Halley, whose motion, on the contrary, is directed from east to west. The science of celestial movements, and that of cosmogony, are equally interested in the solution of the problem to which I allude.

We do not yet know with certainty if comets are of themselves luminous, or if they borrow from the sun all the light with which they shine. The investigation of their phases seemed to be the only means of resolving the question, but hitherto it has entirely failed. Comparative measures of the intensity of light, photometric measures, may lead us to the result in a manner not less incontestible. This kind of observation will doubtless excite the attention of astronomers, during the actual appearance of Halley's comet. The second edition of the Annuaire of 1832 proves to amateurs of the science, that they even, with very feeble instruments, may usefully assist in this curious research.

In the year 1305, Halley's comet had extraordinary brightness; in 1456 it dragged after it a tail which embraced two thirds of the space between the horizon and the zenith; in 1682, though notably less brilliant than in 1305 or 1456, it was classed among the brilliant comets, and its tail was still 30′; in 1759, its appearance would certainly have occupied the attention of none but astronomers, if it had not been the first comet announced a long time previously. These facts seem to prove that comets are diminishing in brightness, and we might be tempted to search for the physical cause in the matter which near the perihelion separates itself from the nebulosity to form the tail, and which the comet seems to disseminate in space. M. Olbers, certainly one of the most competent judges in a subject of this kind, does not regard the gradual enfeebling of comets as proved; he believes that the diminution observed in that of Halley from 1305 to 1456, from 1456 to 1682, and from 1682 to 1759, is only apparent; that it can be explained by the very peculiar relative positions of the sun, the comet, and the earth; and finally, he cites in support of his opinion, the appearance of 1507, intermediate between those whose dates I have mentioned, adducing which for the positions analogous to those of 1759, the comet, according to Kepler, presented nothing remarkable in its intensity. In this state of the problem every one must perceive the interest which must be attached to the circumstances attending the appearance of the comet next November.

It will be on August 20. 1835, near of Taurus; August 28, between Gemini; September 21, in the Coachman; OctcVOL. XIX. NO. XXX VII.-JULY 1835.

C

ber 3, in the Lynx; October 6, in the Great Bear; October 11, the same; October 12, in Arctophylax; October 13, in the Crown; October 15, between Hercules and Serpentarius; October 19, in Ophincus; October 31, the same; November 16, near, of Ophincus; December 26, in the Scorpion, near Antares.

It would be difficult to say with certainty on what day the comet will become visible. The state of the heaven, the power of the instruments, the goodness of the power of sight of the observer, united to physical causes, already mentioned, variations of intensity, render all attempts to solve the problem entirely illusory. M. Olbers believes that at its maximum of lustre, the expected comet, far from surpassing, as has been pretended, that of 1811, will not equal the third comet of 1825, which the public allowed to pass without paying any attention to it.

It is towards the middle of October, that the comet of Halley, during its next appearance, will be nearest the Earth. We may add that its distance will never be less than eight millions of leagues, of twenty-five to the degree. Thus those who have not been entirely reassured by the numerous pleadings published lately in favour of comets, cannot in the present case have any plausible cause of uneasiness.

On the periods of the Deluges of Deucalion, Ogyges, and Noah, according to the various calculations of time of different Nations. By Dr VON SCHUBert.

THE history of nations and countries speaks distinctly of several partial floods and deluges which seem to have been produced by lakes at a high elevation bursting through their barriers. The flood of Deucalion, and also that of Ogyges, have been considered as having owed their origin to the outbreak of highly situated masses of water (as, for example, of the Thessalian lake), and they are supposed to have taken place several centuries after the great Noachian Deluge. A more accurate comparison of the various chronological dates, shews that the ancient calculators of time understood no other than the flood of Noah by the floods of Ogyges and Deucalion. This was in