IC>8 ON COMETS. 



of a long ellipse or parabola as the possible form of * 

 comet's orbit ; and Dorfel, a German astronomer ii. 

 1 68 1, upon a careful consideration of all the observa- 

 tions of the great comet of 1680, came to the positive 

 conclusion that that comet did really move in a parabolic 

 orbit with the sun in its focus. This was an immense 

 step ; but neither Dorfel nor any one else could at that 

 time give any account of the reason why this should be 

 the case, or in what manner the comet was made to con- 

 form its sweep through space in so singular a way to the 

 sun. 



(17.) The wonderful discoveries of Sir Isaac Newton 

 made all this clear. He first showed that the sun controls 

 the movements of these wanderers by the very same force 

 acting according to the very same law which retains the 

 planets in their paths that marvellous law of gravita- 

 tion the same power which draws a stone thrown from 

 the hand back to the earth (in a parabolic curve) 

 which keeps the moon from flying off, and holds her to 

 us as a companion which keeps the planets in their 

 circles, or rather ellipses, about the sun and which we 

 now know holds together several of the stars in couples, 

 circulating one about the other. 



(18.) The great comet of 1680, which occurred while 

 Newton was brooding over these grand ideas which 

 broke upon the world like the dawn of a new day in his 

 "Principia," afforded him a beautiful occasion to test 

 the truth of his gravitation theory by the most extreme 

 case which could be proposed. The planets were tame 

 and gentle things to deal with. A little tightening of 



