152 LIGHT SCIENCE FOR LEISURE HOURS. 



comet when passing away from the sun increased in 

 volume upwards of fortyfold in a single week. 



The tremendous heat to which many comets are sub- 

 jected during perihelion passage is an important point 

 for consideration, in attempting to form an opinion of 

 the physical structure of comets. Newton calculated 

 that the comet of 1680 was subjected to a heat 2,000 

 times greater than that of red-hot iron. But comets 

 have been known to approach the sun even more closely. 

 Sir John Herschel estimates that the comet of 1843 

 was subjected to a heat exceeding in the proportion of 

 24^ to 1 the heat concentrated in the focus of Perkins' 

 great lens. Yet the heat thus concentrated had suf- 

 ficed to melt agate, rock-crystal, and cornelian. 



We cannot wonder that so great an intensity of heat 

 should have produced remarkable effects upon many 

 comets. The great wonder is that any comet should 

 resist the effects of such heat without being dissipated 

 into space. 



We learn from Seneca that Ephorus, an ancient 

 Greek author, mentions a comet which divided into 

 two distinct comets. Kepler considered that two 

 comets which were seen together in 1618 had been 

 produced by the division of a single comet. Cysatus 

 noticed that the great comet of 1618 showed obvious 

 signs of a tendency to break up into fragments. This 

 comet when first seen appeared as a circular nebulous 

 cloud. A few weeks later it seemed to be divided into 

 several distinct cloudlike masses. On December 20 

 6 it resembled a multitude of small stars.' 



