COLLISION OF A COMET AND THE EARTH. 



.? country over winch the Harmattan blows, for more than a 

 hundred leagues, is a series of extensive plains covered with 

 verdure, with a few patches of wood here and there, and inter- 

 sected by a few rivers, with some small lakes. 



13. Various phenomena have raised the question whether at any 

 remote epoch of its physical history the earth was ever struck 

 by the solid nucleus of a comet. 



^Ve have already stated ~ the circumstances which render it 

 highly probable that the comets generally are mere masses of 

 aeriform or vaporous matter. Nevertheless, although this be 

 certain as respects the large majority of these bodies, some among 

 them, more especially those which appeared at remote dates, have 

 had a splendour which it would be difficult to imagine to be pro- 

 duced by the reflection of the sun's light by mere vaporous matter; 

 and even in modern times, since the instruments of observation 

 have been improved, and observers have increased in zeal, activity, 

 and vigilance, and have been greatly multiplied in number, 

 appearances of a nucleus have been observed which some astro- 

 nomers have considered to afford pretty conclusive evidence of the 

 existence of a solid nucleus within the nebulous envelope ; and 

 although many entertain doubts of this, it cannot be said that the 

 existence of a solid nucleus in some of the many comets which 

 have passed through the system is absolutely disproved. 



Assuming, then, the possible existence of a solid comet, and 

 considering the possible (however improbable) eventuality of such 

 a body and the earth passing at the same moment through the 

 same point of space, it may be reasonably asked, 



What would be the consequences of such a catastrophe? 



It must be observed, in the first place, that admitting the bare 

 possibility of certain comets having a solid nucleus, such a mass 

 must be less, incomparably, than the smallest body of the solar 

 system. The grounds upon which this inference rests, have been 

 already stated. 



Now, assuming the earth to move round the sun, and at the 

 same time to have a diurnal rotation upon a certain diameter as 

 its axis, let us see what would happen if it were to receive sud- 

 denly a blow, from a much smaller solid mass encountering it. 



If, in case of such an event, the earth had no previous motion 

 of rotation, and if, as would probably happen, the direction of tho 

 blow given to it did not pass through its centre, it would receive 

 a motion of rotation round an axis at right angles to the plane 

 <Lrawn through the direction of the blow and the centre of the 

 earth, and the time of rotation would depend on the distance of 

 the centre of the earth from the direction of the blow. 



If, however, the earth, before receiving the blow, had already a 



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