570 THE PRINCIPLES OF SCIENCE. [CHAP. 



to realms of new discovery. Take the instance adduced by 

 Herschel. The theory of Newton and Halley concerning 

 comets was that they were gravitating bodies revolving 

 round the sun in elliptic orbits, and the return of Halley's 

 Comet, in 1758, verified this theory. But, when accurate 

 observations of Encke's Comet came to be made, the veri- 

 fication was not found to be exact. Encke's Comet returned 

 each time a little sooner than it ought to do, the period 

 regularly decreasing from 121279 days, between 1786 and 

 1789, to 1210*44 between 1855 and 1858; and the hypo- 

 thesis has been started that there is a resisting medium 

 filling the space through which the comet passes. This 

 hypothesis is a deus ex machind for explaining this solitary 

 phenomenon, and cannot possess much probability unless 

 it can be shown that other phenomena are deducible from it. 

 Many persons have identified this medium with that through 

 which light undulations pass, but I am not aware that 

 there is anything in the undulatory theory of light to show 

 that the medium would offer resistance to a moving body. 

 If Professor Balfour Stewart can prove that a rotating disc 

 would experience resistance in a vacuous receiver, here is 

 an experimental fact which distinctly supports the hypo- 

 thesis. But in the mean time it is open to question 

 whether other known agents, for instance electricity, may 

 not be brought in, and I have tried to show that if, as is 

 believed, the tail of a comet is an electrical phenomenon, 

 it is a necessary result of the conservation of energy 

 that the comet shall exhibit a loss of energy manifested 

 in a diminution of its mean distance from the sun 

 and its period of revolution. 1 It should be added that if 



1 Proceedings of the Manchester Literary and Philosophical Society, 

 28th November, 1871, vol. xi. p. 33. Since the above remarks were 

 written, Professor Balfour Stewart has pointed out to me his paper 

 in the Proceedings of the Manchester Literary and Philosophical 

 Society for i5th November, 1870 (vol. x. p. 32), in which he shows 

 that a body moving in an enclosure of uniform temperature would 

 probably experience resistance independently of the presence of a 

 ponderable medium, such as gas, between the moving body and the 

 enclosure. The proof is founded on the theory of the dissipation of 

 energy, and this view is said to be accepted by Professors Thomson and 

 Tait. The enclosure is used in this case by Professor Stewnrt simply 

 as a means of obtaining a proof, just as it was used by him on a 

 previous occasion to obtain a proof of certain consequences of the 



