304 NOTES. 



of exposing the plate to the rays passing through a very 

 small hole into a dark room, and which form the image, 

 more or less distinct, of external objects. It is unfortunate 

 that this did not appear in the paper of 1796, "because there 

 can be little doubt that it would have led to making trials 

 which must have ended in the discovery of the photographic 

 process many years before it was eventually introduced. 



NOTE V., p. 226. 



The subsequent examination of the question touching the 

 origin of aerolites appears to have thrown great light upon 

 the subject, and may be said to have displaced the lunar 

 theory. Laplace, Biot, and Poisson investigated the sub- 

 ject of the initial velocity required to project a body from 

 the Moon to the Earth. Laplace made this 7,379 French feet 

 in a second; Poisson, 7,123; Biot, 7,791; Olbers made it 

 7,780 ; and these numbers are without making any allowance 

 for atmospheric resistance. But the mean velocity of aerolites 

 is 114,000 ; and therefore the initial velocity is calculated at 

 about 110,000, or fourteen times greater than Laplace's pro- 

 portion, which he reckons at five or six times the velocity of 

 a cannon ball. There is no ground for believing that any 

 volcano exists in the Moon sufficiently active to exert this 

 force. If one does exist it must be of double the force of any 

 known on the earth. Furthermore the aerolites, for the most 

 part, reach the earth moving in one direction ; and there is no 

 reason, on the lunar theory, why they should move in one 

 direction rather than another. The whole subject is treated 

 with great clearness in Humboldt's ' Cosmos ' (vol. i. p. 105), 

 and more particularly in note 69, p. 383, of the admirable 

 translation of that work by General Sabine, who has added 

 some very valuable notes, especially at pp. 411, 454-457. 

 Poisson (Mec. Annal. torn, i.) discusses the lunar theory of 

 aerolites, and rejects it. 



LONDON : paiXTED BY WILLIAM CLOWES ASD SONS, STAMFORD STREET. 



