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tail, follow as a corroUary to the theory I have laid down, 

 and seem to me strongly to indicate that the tail is really a 

 material appendage of the comet, and not an effect produced 

 by it upon any medium through which it may be supposed 

 to move. 



It may be said in objection to my theory that comets are 

 not made up of such chemical substances as I have instanced 

 in the case of the hypothetical planet, to which I would 

 reply, " Nor need they be." The theory in question only 

 requires that they should be composed, at any rate in part, 

 of materials evaporable by heat aud whose vapours are con- 

 densible by cold, and this I think, apart from being an 

 almost self-evident proposition, the spectroscope has shown 

 to be a fact in the instances of the small comets examined 

 by its aid. It indicates, as I understand, the existence of 

 heated gaseous matter about the nucleus, and of liquid or 

 solid material in a state of infinitesimal division in the sub- 

 stance of the tail. 



The six-tailed comet of 1744 will, I have no doubt, be 

 pointed to as one whose phenomena it is difficult to explain 

 in accordance with the theory I have advanced. I would 

 ask those who feel disposed to raise this objection to examine 

 the evidence upon which it is affirmed that the comet in 

 question was really possessed of a multiple tail. To my 

 own thinking that evidence is so far from being conclusive 

 that it would be premature to offer an explanation of the 

 phenomenon before the appearance of another comet, unmis- 

 takably presenting the peculiarities attributed to that of 

 1744. 



There are instances on reliable record in which comets 

 have been known to present two tails curved in opposite 



