252 THE POPULAR SCIENCE MONTHLY. 



comet would cross the earth's path in that year, although it was ex- 

 plained 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 as- 

 tronomers 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 w^as 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 ap- 

 pendages, 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 attend- 

 ing on the comet, though it is certain that not one of the known long- 

 tailed comets can have kept its stratified meteoric tail (assuming al- 

 ways 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 ichen 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 



