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of their existence. That precipitation should occur behind the 
head of the comet or in the space occupied by the head’s shadow, 
it is only necessary to assume that the sun’s calorific rays are 
absorbed more copiously by the head and nucleus than the 
actinic rays. This augments the relative superiority of the 
actinic rays behind the head, and enables them to bring down 
the cloud which constitutes the comet’s tail. 
5. The old tail, as it ceases to be screened by the nucleus, 
is dissipated by the solar heat; but its dissipation is not instan- 
taneous. The tail leans toward that portion of space last quitted 
by the comet, a fact of observation being thus accounted for. 
6. In the struggle for mastery of the two classes of rays, a 
temporary advantage, owing to variations of density or some 
other causes, may be gained by the actinic rays even in parts of 
the cometary atmosphere which are unscreened by the nucleus. 
Occasional lateral streamers and the apparent emission of feeble 
tails towards the sun would be thus accounted for. 
7. The shrinking of the head in the vicinity of the sun is 
caused by the beating against it of the calorific waves, which 
dissipate its attenuated fringe, and cause its apparent contrac- 
tion. 
At the conclusion of the lecture, which was listened to with 
the greatest attention by a large audience, the President (Rev. 
Professor Selwyn) conveyed the thanks of the Society to the 
lecturer, who briefly acknowledged the compliment, and ex- 
pressed his pleasure in being able to appear before the 
Society. 
