THE SIDEREAL UNIVERSE. 727 



But even supposing that comets are not so marvellously 

 light as to allow of a man carrying them oh upon his shoul- 

 ders without being as strong as Atlas, and if, too, their 

 shock is far from being so formidable as Buff on supposed, 

 yet certainly these bodies are too imperfectly known for us 

 to lay down general rules about them. M. Guillemin, in his 

 remarkable work on the heavens, speaks as follows : " If 

 there be comets the nebulosity of which is quite gaseous, 

 and so transparent that small stars can be seen through 

 their substance, there are others the nucleus of which is, 

 without doubt, very dense, as their light was bright enough 

 to be perceptible in full day, even in the vicinity of the 

 sun." 



The mass of Donati's comet has been estimated at about 

 the seven-hundredth of that of the earth ; " that is to say," 

 says M. Faye, " the same weight as a sea of 16,000 square 

 leagues surface, and 330 feet in depth. It must therefore 

 be admitted that such a mass, impelled with great speed, 

 might produce sensible effects by coming in contact with 

 the earth." 



Is it not possible, in cases where the tail of a comet is 

 formed of atoms widely scattered, that the brilliancy of 

 their nucleus may just be the result of incandescence ? And 

 then, even supposing there was nothing to fear from the 

 shock, would not the approach of such a furnace be enough 

 to make us dread being burned up ? 



The phenomenon of shooting-stars strikes the untutored 

 mind less than the appearance of comets, and yet, notwith- 

 standing its frequency, the explanation of it is not free from 

 obscurity on some points. 



