Page:Popular Science Monthly Volume 16.djvu/266

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252
THE POPULAR SCIENCE MONTHLY.

comet would cross the earth's path in that year, although it was explained that the comet would pass a month before the earth reached that point of her path. "We escaped that time," Sir John Herschel wrote in 1866. "Had a meeting taken place, from what we know of comets, it is probable that no harm would have happened, and that nobody would have known anything about it." But from what we have since learned we have reason to believe that we should have known a great deal about the encounter, though it remains altogether probable that no harm would have happened. For we have learned that as a rule the tracks of comets are followed by millions of meteoric bodies, which, as the earth passes through the flight, produce displays of falling stars, each meteor in its rush through the earth's atmosphere producing a trail or streak of light; and doubtless in the head itself of a comet meteoric bodies are much more richly strewed, so that an encounter with the head would produce an unusually splendid display of falling stars. It is, however, very noteworthy, as will presently appear more clearly, that no display of meteors is recorded as having occurred in the last week of November, 1832, though the comet had crossed the earth's track less than a month before. Yet in 1872 astronomers were led to expect somewhat confidently that, as the earth passed the track of Biela's comet, which had gone that way only some ten or twelve weeks before, there would be a shower of falling stars produced by the bodies following in the comet's path.

I may pause here, by the way, to remark on the clear way in which this expectation, and what was actually observed, should show every one who has clear mathematical conceptions that it is the train, and not the tail, of a comet, which is followed by meteoric attendants. Professor Tait, of Edinburgh, who is a master of mathematical analysis, but apparently wanting in the power of clearly conceiving geometrical relations, has based on the mistaken idea that comets' tails are made up of meteors a wild theory of the phenomena presented by these appendages, a theory which could not be accepted even if it had been proved that comets' tails are formed of meteor-flights. For he explains the appearance of a long cometic tail as due to the circumstance that at the time the earth is in the plane of a vast meteoric stratum attending on the comet, though it is certain that not one of the known long-tailed comets can have kept its stratified meteoric tail (assuming always that it had one) directed with its plane earthward during half the time of the tail's actual visibility. But so far as real evidence is concerned, the probability is that there are no meteors in or near the tail of a comet. For, on the one hand, on the only occasion when the earth is known to have passed through the tail of a comet—namely, when she passed through the tail of the splendid comet of 1861—no meteors were seen which could have belonged to that appendage; and on the other, in every single case in which meteors have been associated with a comet, those meteors have not been in or anywhere near the