.

Concerning the physical constitution of comets we have but a very imperfect knowledge at present. Sir John Herschel regards them as masses of thin vapour, capable of reflecting the solar rays from their internal as well as external parts-an inference which is rendered necessary, in order to account for all the phenomena revealed by telescopes. It is certain that stars of a very faint class have been repeatedly seen through comets of from fifty thousand to one hundred thousand miles in diameter, and, in the majority of cases, not the least perceptible diminution of the star's brightness took place. There are one or two instances on record, where astronomers have been convinced of a sensible increase of brilliancy when a star has been viewed through the cometic vapour. In addition to a remarkable observation of this kind by Piazzi, at Palermo, during the appearance of the grand comet of 1811, we may mention a more recent one by Professor Reslhuber of Kremsmünster, in reference to a star seen through the denser part of a comet discovered by M. Brorsen in March 1846, and which, under ordinary circumstances, belonged to the eighth class, or was just beyond unassisted vision. When the star was centrally covered by the comet, it became very considerably brighter, and was judged to be equivalent to a star of the sixth magnitude, in which case it would have been distinctly visible without a telescope. Professor Struve made some interesting observations during the transit of the comet of Biela over a small star, on November 6th, 1832. The brightness of the star was not in the least diminished by the intervention of the comet, and its light suffered no perceptible

refraction, a point established by a continuous series of measures with the micrometer.

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That comets shine by a reflected light is a conclusion which few astronomers will dispute. It is evident from telescopic observation of the degree of brilliancy at different distances, and in various positions of a comet, with respect to the sun and eart and M. Arago has established the fact from experiments with a polariscope, during the visibility of the famous comet of Halley in the autumn of 1835 Still the variation in the intensity of light is not un versally such, as should follow if the comet mere. reflected the sun's rays, under certain permaneL. conditions, and we are under the necessity of looking to physical causes inherent in the body itself for an explanation of some few observations which appear irreconcilable with the theory of reflected solar light. The first comet of 1780 was closely examined by Dr. Olbers, the eminent astronomer of Bremen. He found it attained its greatest brightness on the 8th of November, thirteen days subsequent to its discovery. whereas, according to the law of reflected light, it should have become gradually fainter since the first observation; and, supposing the comet self-luminous, the intensity of light should have increased each day until November 26, when the maximum would take place yet, in the interval between the 8th and 26th of that month, it grew rapidly less. The comet discovered by Dr. Galle, of Berlin, on the 25th of January, 1840, presented similar anomalies. Assuming that it had no light of its own, it should have appeared twice as bright on the 23rd of February as on the 21st

of March, yet at the latter date Professor Plantamour found the intensity of light had increased in the proportion of more than two to one. Such variations are probably to be attributed to changes in the physial constitution of the comet, due to the action of the

sun.

With our present imperfect knowledge of the nature of the matter of which these bodies are composed, it is not to be expected that we can gain a clear insight into the laws of the forces exercised upon it, through which the envelope and tail are formed. The nucleus,

more condensed part of the head, appears to possess the power of throwing off towards the sun a portion of the cometic atmosphere, which, before it can attain any great distance from the nucleus, is driven backward in two streams passing on either side of the head, and ultimately blending into one to form the tail. This repulsive energy must very far exceed the force of gravitation. Generally speaking, the axis of the tail preserves a rectilinear form throughout the greater part of its length, a curvature of the extremity being only occasionally seen, and probably attributable to the failure of the repulsive force, and the gradual effect of a resisting medium upon these distant and extremely rare portions of the train. The disappearance of the tail as the comet recedes from the sun, may be owing either to its being attracted into the nucleus as that luminary loses its power upon it, or it may be partly dispersed in the surrounding space.

In the case of bodies like comets, moving through the planetary spaces in every direction, it cannot

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be denied that there is a possibility of the Earth's coming in collision with one of them in the lapse of time; but we are able to show from legitimate reasoning that the chance of such a catastrophe is very small indeed. M. Arago has calculated that the probability against it is greater than 250,000,000 to one. We know that the earth has had one or two narrow escapes within the last two centuries, as in 1680 and 1832, when comets crossed the plane of the ecliptic almost on the path of our globe, though at these times we were, perhaps fortunately, removed many millions of miles from the dangerous part of the orbit. The comet of 1770 has approached nearer to us than any other of these bodies whose elements have been sufficiently well determined. On the 1st of July it was distant from the Earth only 363 terrestrial semidiameters or 1,438,000 miles. The great comets of 837, 1402, and 1472, must have come within. a very short distance from our globe, and another small one, which was observed by Flaugergues in 1826, seems to have made a close appulse. Olbers mentions several which have crossed the ecliptic at points not far removed from the annual track of the earth.

The comet of 1684 approached it within 216 terrestl.semidiamtrs.

CHAPTER III.

OF THE PATHS OF COMETS IN SPACE-ELEMENTS OF THEIR ORBITS-USES OF A TABLE OF ELEMENTS— EFFECTS OF PLANETARY ATTRACTION UPON THEIR MOVEMENTS.

THE curve described by the generality of comets is in all probability a very excentrical ellipsis, so nearly approaching a parabola, that for those parts of the orbit near the perihelion, where alone the comet is visible from the earth, no sensible difference is caused by the substitution of the latter curve; and accordingly, as the calculation in the parabola is so much easier and shorter than in the ellipse, astronomers always employ it to represent the paths of comets in the heavens, and to predict their future positions as viewed from the earth or sun. When one of these bodies has been discovered, and several observations (not less than three) have been obtained, they are submitted to calculation; and the elements of the orbit, as they are termed, are found on the supposition that the real path through space is in a parabola with these elements we can find for any particular time, the comet's distance from the earth, and its situation in our heavens, and, in fact, trace its C 2