Their numbers are very great; several have been seen by the naked eye, and the telescope has increased the number to hundreds. What they are, or what purpose they fulfil in the economy of nature, is unknown. That they are material bodies of some sort, is known from their being attracted by other bodies near which they pass. A few of them are considered as permanent members of the system, as they return at certain periods to the sun, moving in very elliptic orbits, some of which are known. The comet which appeared in 1835, afforded all an opportunity of seeing it. The important point, relative to this, was that its orbit is known, and the interval between its periodical returns has been calculated. Comets of much greater magnitude and more splendid appearance have been seen, yet this, as it relates to astronomy, is the most important of all. The honor of first predicting its return belongs to Dr. Halley, a friend and contemporary of Sir Isaac Newton.
What can be the use or nature of these bodies, is a question which naturally occurs, but to which science can yet give no satisfactory answer; their peculiarities being subjects of much greater wonder to the astronomer, than to the children who gaze on them. They are material bodies, it is true, yet composed of such materials as almost to leave one in doubt as to their deserving the name.
The younger Herschel says that “the most unsubstantial clouds that float in the higher regions of the atmosphere, may be looked upon as dense and massive bodies, compared with the filmy and all but spiritual texture of a comet.” It is even probable that the substance is not to be compared for density with the air we breathe; that if a volume of cometary matter could be weighed, it would not be found so heavy as an equal volume of atmosphere.
I think you will understand some of the reasons that have led to such a conclusion. If you observe smoke, steam, or any vapor, you will find that the farther you are from it, the more difficult it is to see through it. The fog which hides the distant mountains, or even the sun, does not obscure those objects immediately about you; the thin vapor which you see at a distance covering the meadow, disappears on approaching it. A body of vapor, therefore, becomes at a distance perfectly opaque and impenetrable to the direct rays of the sun; so, were the planet Mercury composed of materials of a like nature and density, they would still appear, during a transit, like a black spot on the sun’s disc. The comet, however, instead of obstructing the rays of the sun, offers little impediment to those of a star. No vapory substance perceptible to our senses could maintain any degree of transparency at a comet’s distance, but, if the volume were large enough, would appear as a perfectly dense and opaque mass; yet during the visit of the comet of 1835, a star of about the tenth magnitude was seen through the most dense part of it, within one three hundred and sixtieth part of a degree, of its centre. Through the centres of other comets, stars have been seen, so small that it would require a thousand to make one as large as the smallest of those seen by the naked eye. The conclusion, then, is reasonable enough, that were thousands of these bodies placed between us and the sun, it would obstruct no appreciable portion of its rays, and that it would continue to appear in undiminished splendor.
We shall have a little more to say about comets in our next number.
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