xxv.] ACCORDANCE OF THEORIES. 571 



Professor Tait's theory be correct, as seems very probable, 

 and comets consist of swarms of small meteors, there is no 

 difficulty in accounting for the retardation. It has long 

 been known that a collection of small bodies travelling 

 together in an orbit round a central body will tend to fall 

 towards it. In either case, then, this residual phenomenon 

 seems likely to be reconciled with known laws of nature. 



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 

 atmosphere at the earth's surface would be 968 feet per 

 second, and rude experiments made by him in the cloisters 

 of Trinity College seemed to show that this was not far 

 from the truth. Subsequently it was ascertained by other 

 experimentalists that the velocity of sound was more 

 nearly 1,142 feet, and the discrepancy being one-sixth 

 part of the whole was far too much to attribute to casual 

 errors in the numerical data. Newton attempted to 

 explain away this discrepancy by hypotheses as to the 

 reactions of the molecules of air, but without success. 



New investigations having been made from time to time 

 concerning the velocity of sound, both as observed experi- 

 mentally and as calculated from theory, it was found that 

 each of Newton's results was inaccurate, the theoretical 

 velocity being 916 feet per second, and the real velocity 

 about 1,090 feet. The discrepancy, nevertheless, remained 

 as serious as ever, and it was not until the year 1816 that 

 Laplace showed it to be due to the heat developed by the 

 sudden compression of the air in the passage of the wave, 

 this heat having the effect of increasing the elasticity of 

 the air and accelerating the impulse. It is now perceived 



Theory of Exchanges. He is of opinion that in both of these 

 cases when once the proof has been obtained, the enclosure may be 

 dispensed with. We know, for instance, that the relation between the 

 inductive and absorptive powers of bodies although this relation 

 may have been proved by means of an enclosure, does not depend 

 upon its presence, and Professor Stewart thinks that in like manner 

 two bodies, or at least two bodies possessing heat such as the sun 

 and the earth in motion relative to each other, will have the differ- 

 ential motion retarded until perhaps it is ultimately destroyed. 



