xxv.] ACCORDANCE OF THEORIES. 557 



vation could yield, so that the theory could hardly be said 

 to admit of direct verification. 



The specific heat of air was believed on the grounds of 

 direct experiment to amount to 0-2669, the specific heat of 

 water being taken as unity ; but the methods of experi- 

 ment were open to considerable causes of error. Eankine 

 showed in 1850 that it was possible to calculate from the 

 mechanical equivalent of heat and other thermodynamic 

 data, what this number should be, and he found it to be 

 0-2378. This determination was at the time accepted as 

 the most satisfactory result, although not verified ; sub- 

 sequently in 1853 Kegnault obtained by direct experiment 

 the number 0-2377, proving that the prediction had been 

 well grounded. 



It is readily seen that in quantitative questions verifi- 

 cation is a matter of degree and probability. A less 

 accurate method of measurement cannot verify the results 

 of a more accurate method, so that if we arrive at a 

 determination of the same physical quantity in several 

 distinct modes it is often a delicate matter to decide which 

 result is most reliable, and should be used for the indirect 

 determination of other quantities. For instance, Joule's 

 and Thomson's ingenious experiments upon the thermal 

 phenomena of fluids in motion l involved, as one physical 

 constant, the mechanical equivalent of heat ; if requisite, 

 then, they might have been used to determine that im- 

 portant constant. But if more direct methods of experi- 

 ment give the mechanical equivalent of heat with superior 

 accuracy, then the experiments on fluids will be turned to 

 a better use in determining various quantities relating to 

 the theory of fluids. We will further consider questions 

 of this kind in succeeding sections. 



There are of course many quantities assigned on theo- 

 retical grounds which we are quite unable to verify with 

 corresponding accuracy. The thickness of a film of gold 

 leaf, the average depths of the oceans, the velocity of a 

 star's approach to or regression from the earth as inferred 

 from spectroscopic data (pp. 296-99), are cases in point ; 

 but many others might be quoted where direct verifica- 

 tion seems impossible. Newton and subsequent physicists 



1 Philosophical Transactions (1854), vol. cxliv. p. 364. 



