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tail, follow as a corrollary 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 and 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 1741 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 
