On the Formation of the Tails of Comets. 105 



ture of comets. 2. Their structure. 3. Their variations in form 

 and dimensions. 



1. The nature of Comets. — It is well known that it has long 

 been considered an established fact in astronomical science, that 

 the cometary bodies are collections of matter which is far more 

 subtil in its texture than the lightest gas, or the most evanescent 

 vapor. There are visionary theorists, however, who, not content 

 with the mere shadow of materiality which astronomers have 

 attributed to these bodies, deny them the honors of material ex- 

 istence altogether, seeing in them nothing but fortuitous images. 

 And there are men of science, who with Appian and Tycho 

 Brahe, of old, while they conceive the head to be a real corpore- 

 ity, maintain that the tail is a mere spectre of light. This latter 

 theory has, it must be allowed, at the first glance, an air of great 

 plausibility; as it is remarkably simple in its conception, and 

 seems to give a ready explanation of the more familiar phenome- 

 na. But it would be easy to show that it does not stand the test 

 of comparison with facts, when attentively applied. It would be 

 a work of supererogation, however, to attempt this in the present 

 communication, as there is no evidence that this theory has secur- 

 ed any foothold in the region of true science. I shall accordingly 

 take it to be an admitted fact that the tail of a comet, as well as 

 its head, is a material substance. 



Judging from appearances, there is, moreover, no distinction of 

 kind in the matter of which comets are composed. All parts of 

 a comet shine with the same nebulous light, with the exception 

 only of the nucleus; and this often assumes a similar appearance, 

 when viewed through a telescope. If any further evidence be 

 wanted of an identity of nature in the matter of the nucleus and 

 of other parts of the comet, we may find it in the undoubted fact 

 that the nebulous envelope of the head receives frequent supplies 

 from the nucleus. It is not to be overlooked, although, that all 

 this evidence really applies, at least directly, only to the outer 

 parts of the nucleus. The central portions, for any direct proof 

 to the contrary that we have, may be regarded as unchangeably 

 solid, as they have been by Schrceter and some other astronomers. 

 But the existence of comets with very small nuclei and of others 

 with no nucleus at all, furnishes a strong presumption, in confir- 

 mation of the natural inference from what is above stated, that 



Vol. xl vi, No. 1.— Oct.-Dec. 1843. 14 



