ON COMETS. . Ill 



90 millions of miles, so that it must have been shot out 

 with immense force in a direction from the sun, a force 

 far greater than that with which the sun acted on and con- 

 trolled the head of the comet itself, which, as the reader 

 will have observed, took from November 10 to December 8, 

 or 28 days, to fall to the sun from the same distance, and 

 that with all the velocity it had on November 10 to start 

 with. 



(20.) All this is very mysterious. We shall never perhaps 

 quite understand it, but the mystery will be at all events 

 a little diminished when we shall have described some of 

 the things which are seen to be going on in the heads 

 of comets under the excitement of the sun's action, and 

 when calming and quieting down afterwards. At pre- 

 sent, however, we must get on with another part of our 

 subject. 



(21.) Only two years after this appeared another bril- 

 liant comet, and our countryman, Edmund Halley, fol- 

 lowing Newton's example and employing his system of 

 calculation, computed its orbit, assuming (which simpli- 

 fies the calculation very much) that orbit to be a para- 

 bola. He found its path to be very different from that 

 of Newton's comet. Instead of nearly grazing the sur- 

 face of the sun, its nearest approach to it was about 55 

 millions of miles, or about half-way between the orbits of 

 Mercury and Venus. The plane of its motion, too, was 

 much less inclined to that of the planets' orbit or the 

 ecliptic viz., about i7J, and its motion was not direct, 

 as Newton's was, but retrograde. 



(22.) Halley was encouraged by the good agreement of 



