ACCORDANCE OF QUANTITATIVE THEORIES, &c. 213 



tween 1855 and 1858. The theory of gravitation alone 

 cannot account for such a continued decrease of period ; 

 hence the hypothesis 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 pos- 

 sess any validity or probability unless it can be shown 

 that other phenomena are deducible from it. Many per- 

 sons have identified this medium with that through which 

 heat undulations pass, but I am not aware that there is 

 anything in the undulatory theory of light to show that 

 the medium would oifer resistance to a moving body. If 

 Professor Balfour Stewart can prove that a rotating disc 

 experiences resistance even in a perfectly vacuous receiver, 

 here is an experimental fact which distinctly supports the 

 hypothesis. 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 

 seems highly probable, on other grounds, the tail of a 

 comet is an electrical phenomenon, it is almost a neces- 

 sary result of the theory 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 k . If so, the residual phenomenon 

 seems to confirm an hypothesis as to the nature of the 

 comet itself, rather than that of the medium through 

 which it moves. 



In other cases residual phenomena have involved im- 

 portant inferences not recognised at the time. Newton 

 showed how the velocity of sound in the atmosphere 

 could be calculated by a theory of pulses or undulations 

 from the observed tension and density of the air. He 

 inferred that the velocity in the ordinary state of the 



k ' Proceedings of the Manchester Literary and Philosophical Society/ 

 28th November 1871, vol. xi. p. 33. 



