POSSIBLE COLLISION WITH THE EAETH. 



between the two points "where they are nearest is less than the 

 semi-diameter of the comet, and it follows, consequently, that if 

 the earth and comet were to arrive together at these points, the 

 earth must pass through the comet. If the comet were solid, 

 which it is not, a collision must take place. But being composed 

 of the lightest and most attenuated vaporous matter, the effect 

 would be the same as if the earth were to pass through a very thin 

 cloud. 



This particular comet happens to be one of the few which have 

 been ascertained to revolve round the sun in a definite period like 

 the planets, with this difference, however, that the orbit is a 

 somewhat elongated oval instead of cne which is nearly circular. 

 The period of this comet being about six years and three quarters, 

 it follows that it must pass through the place of danger to the 

 earth once in that interval. 



It passed through that place in 1832, under circumstances which 

 excited among the world in general, who were taught to expect 

 its approach, and to know its proximity to the earth's path, a 

 certain panic of apprehension as to the possible consequences. 

 These fears were however groundless, for the comet passed through 

 the point of danger on the 29th October, and the earth did not 

 arrive at that point until the 30th November. Now, since the 

 earth moves at the rate of above a million and a half of miles per 

 day, it follows that on the 29th October, the day on which the 

 comet passed through the point of danger, the earth must have 

 been nearly fifty millions of miles from that point. 



In 1805, the same comet passed through the same point, under 

 circumstances which, had they been as generally known as in 

 1832, might have more reasonably excited apprehension, for in 

 that case the distance of the earth from the comet was only five 

 millions of miles. 



It may, nevertheless, be observed with truth, that although 

 the danger of an encounter with the comets whose orbits are 

 known, be insignificant, the risk with relation to the far more 

 numerous class of these bodies, whose motions are unascertained 

 and which pass continually among the planets may be much 

 greater. 



Nothing, however, is more easy than to apply to this question 

 the well understood principles of the theoiy of probabilities, 

 assuming such conditions respecting the number and magnitude 

 of the comets, as all must admit to be the most favourable imagi- 

 nable to the catastrophe of collision. This has been accordingly 

 done. It has been shown that, assuming the number of comets 

 which pass within the earth's orbit to be the greatest that it can 

 be imagined to be, and that the magnitudes of these comets be 



73 



