Fargcrovan.— On the Constitution of Comets. 479 
b 
considerable solid nuclei, calculated the heat 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 caleulated 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 débris 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! Buta 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 oceulted 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. 
