Lesson xn.] COMETS. 53 



another revolution through its orbit; and, of course, 

 their periods are not precisely ascertained. 



The tail of a Comet, which is generally its most 

 conspicuous part, is thought by NEWTON to be a 

 prodigious quantity of fume and vapours, flying 

 oft" from its body, as it becomes more and more 

 heated in its approach to the Sun. These tails 

 arc sometimes exceedingly long, some having been 

 computed to be 80 millions of miles in length : 

 that they are vapours was thought evident from 

 these considerations.; first, the fixed stars are often 

 seen through them ; secondly, they appear broader 

 on their upper part than near the head of the Co- 

 met; thirdly, the tails lie always towards those 

 parts which the Comets have just left ; and fourth- 

 ly, they seem most splendid and large immediately 

 after they return from the Sun; all which is 

 agreeable to the nature of smoke and vapour. 



The hypothesis of NEWTON, as above recited, 

 fcas been objected to by Dr. Hattey and Dr. Ha- 

 milton, who think that the tail of a Comet is 

 formed of matter which has not the power of re- 

 fracting and reflecting the rays of light; but that 

 it is a lucid, or self-shining substance: and from 

 its similarity to the aurora borealis, they think it 

 probable that it is produced by a similar cause, and 

 is properly an electrical phenomenon. 



A more recently advanced opinion is that of Dr. 

 Herschel, who has affirmed, that, after a very dili- 

 gent examination, he could not perceive the least 

 appearance of any solid nucleus ; Comets seeming 

 to be mere collections of vapours condensed about 

 the centre of each. J3e farther supposed that the 

 D 3 orbits 



