with Remarks on the Construction of its different Parts. 139 
That the luminous matter ascending in the hollow cone, 
received no addition to its quantity from any other source than 
the exposed hemisphere, we may conclude from its appear- 
ance ; which notwithstanding the great circumference of the 
cone it filled, at the altitude of 6 degrees from the head, was 
never seen with increased lustre ; although the diameter of an 
annular section of it, in that place, must have been nearly 15 
millions of miles, and was but little more than half a million 
at its rising from the envelope. 
This consideration points out the extreme degree of rare- 
faction of the luminous matter about the end of the tail ; for 
its expansion, while still much confined in the streams, at the 
altitude which has been mentioned, must have exceeded the 
density it had at rising about 524 times ; but when afterwards 
it extended itself so as to produce nearly an evenly scattered 
light over the whole compass of the end of the tail, we may 
easily conceive to what an extreme degree of rareness its 
expansion must have been carried. 
The vacancy occasioned by the escape of the nebulous 
matter, which after rarefaction passed from the hemisphere 
exposed to the sun into the regions of the tail, was probably 
filled up, either by a succession of it from the opposite hemi- 
sphere, or by a rotation of the comet about an axis ; and the 
gradual decomposition of this matter would therefore be car- 
ried on as long as any remained to replace the deficiency. 
That such a kind of process took place, seems to be sup- 
ported by the observations which were made during the re- 
gression of the comet from its perihelion. For the space 
between the branches of the tail, very near the head of the 
comet, became gradually of a darker appearance than before ; 
Ts 
