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Page:Popular Science Monthly Volume 21.djvu/205

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not be very wonderful that it should reappear now after thirty-seven years, instead of one hundred and seventy-five years. The velocity of a body moving in the solar system depends simply on its distance from the sun, and on the major axis of its orbit. If the velocity is reduced by a resisting medium, there will be a reduction of the major axis, and there is nothing whatever unreasonable in the supposition that, however weak the corona may be, its resistance would have a very great effect upon the motion of a comet which rushes through it, so that I should not be at all surprised if it should turn out that this comet of 1880 is the same as the comet of 1843 and that of 1668, and that its revolution has been so much affected that possibly it may return in, say, seventeen years."

These remarks of Mr. Marth were some time since quoted by Mr. Proctor, and made the basis of an article which in unscientific circles produced to some extent a most absurd sensation. Mr. Proctor's remarks on the subject have been misinterpreted as indicating the probable destruction of life upon the earth about the close of the present century. His language, however, though somewhat unguarded, expressed no such opinion.

The three comets named above approached nearer the sun than any other known, except, perhaps, that of 1680. In fact, when nearest the sun they actually grazed the solar atmosphere, or passed through its outermost portions. Now, it is well known that the motion of a planet or comet through a resisting medium continually lessens its distance, and hence accelerates its velocity. Messrs. Marth and Proctor assume that the passage of the comet of 1668 through the outer portions of the sun's atmosphere reduced its previously long but unknown period to one hundred and seventy-five years, so that its next appearance was in 1843. The perihelion distance at that date was still less; the comet met with greater resistance, and the period was shortened to thirty-seven years. The time of revolution would thus be lessened at each successive return, and ultimately the comet would plunge into the sun. Striking the solar surface with a velocity of three hundred and fifty miles a second, the amount of heat produced by the concussion and radiated to the earth might raise the temperature to such a degree as to destroy life upon our planet. Such are the conjectures suggested in Mr. Proctor's paper. Let us briefly consider them.

In the first place, the fact on which the theory of the supposed catastrophe is based—viz., the identity of the three comets is extremely doubtful. It is much more probable, in view of all the circumstances, that they are different bodies moving in similar orbits.

Again, the period of seventeen years, fixing the comet's next return, according to Mr. Marth, about 1897, was the merest conjecture, not founded on any mathematical calculation whatever. It is true that the passage o a comet through the sun's atmosphere would short-