Faieclough. — On the Constitution of Comets. 479 



considerable solid nuclei, calculated the beat to which the body of his comet 

 (1680) was raised, and how long it would take to cool, on the supposition 

 of its being as large as the moon. Another astronomer has calculated the 

 effect upon the earth's orbit of a comet, with three times the mass of the 

 earth, passing within 40,000 miles of her. Maupertuis sought to relieve the 

 popular dread of collision by the suggestion that it might only destroy a 

 part of the terrestrial surface, and that those who survived the shock might 

 find the debris of the comet to consist largely of gold, diamonds, and the 

 like. Boyle, however, the celebrated French philosopher, published a 

 treatise in 1680 setting forth 239 elaborate reasons why comets could 

 neither do or presage evil to the earth. 



The popular idea of a comet, is a star with a tail ! But a tail is only a 

 temporary appendage to those that have it, while multitudes exhibit no tail. 

 A constant characteristic of comets must be sought in the path pursued, 

 rather than in any appearance presented to the eye. Some comets have 

 been seen that could at times only be distinguished from stars by their 

 course. Others present a star-like nucleus, surrounded by a coma, or a 

 vast nebulous atmosphere. Donati's comet (1858) appeared to have a 

 nucleus, or pellet of light, 1,600 miles in diameter. This was surrounded by 

 two envelopes, one 7,000 and the other over 12,000 miles, high. The whole 

 diameter of the head was 26,400 miles. Other comets, again, present only 

 a nebulous mass, somewhat condensed at the centre, owing, probably, to the 

 greater depth of matter. 



Whether the nucleus is solid opaque matter or not, is, perhaps, an open 

 question. Some observers, in the last century, supposed that they saw 

 phases in certain nuclei, similar to those of the moon. But this was, 

 perhaps, the result of earnest expectation and of devotion to a theory. 

 More recent observers have not obtained similar results, and the spectro- 

 scope, as applied to several faint comets, seems to show that the nucleus 

 does not shine by reflected sunlight, but has some apparently native lumin- 

 osity. The tail, however, and the outer envelope of the head shine by 

 reflected light. Some observers declare that they have seen stars through 

 the nucleus, while others say they have seen stars occulted by the nucleus. 

 It must be observed here that a bright nucleus would obliterate a small 

 star by its very lustre, whether transparent or not. Through comets with 

 purely nebulous heads, stars have certainly been seen. Sir J. Herschell 

 declares that he has seen stars of the sixteenth or seventeenth magnitude, 

 that a breath would obscure, through fully 50,000 miles of cometic matter. 

 Compared with this extreme tenuity, this almost spiritual subtilty, the 

 highest and most feathery cirrhus of our atmosphere may be regarded as 

 dense and solid. 



