Mr. Galbraith on the Velocity of Sound. 215 



through the atmosphere, and obtains a result corresponding 

 to the most accurate experiment then known. His notions 

 relative to the crassitude of the solid particles of air, and va- 

 pours floating in the atmosphere, seem rather illogical, and give 

 his reasoning the appearance of an attempt to reconcile his 

 theory with observation upon principles hardly admissible. 

 In fact, the part of the Principia which treats of the nature 

 and velocity of the aerial pulses is confessedly obscure, and 

 the soundness of the reasoning has been called in question by 

 philosophers of the greatest eminence. On this point, indeed, 

 it is difficult to form an opinion entirely free from objection ; 

 and it is agreed on all hands, that the velocity of sound as- 

 signed by Newton is accurate if the law of elasticity on which 

 his investigation proceeds be admitted. 



If these seemingly gratuitous assumptions, to which we have 

 already alluded, be rejected, the velocity of sound determined 

 by experiment is greater than the quantity computed by New- 

 ton's formula by one sixth of the whole, from theory. 



Laplace having considered this subject, conjectured that the 

 true explanation of this great difference was to be found in 

 the law of Boyle and Mariotte ; namely, that the elasticity of 

 air is directly as its density. Now this law is exact only when 

 the temperature remains unchanged. It is admitted that a 

 series of aerial pulses is a succession of condensations and 

 rarefactions, which are always accompanied with the disen- 

 gagement and absorption of heat, attended by a corresponding 

 change in the elasticity of the air. The temperature is, no 

 doubt, soon equalized by the transfer of heat between the 

 atmosphere and the air in motion. But to produce this effect, 

 a sensible time is required ; while the elastic force is exerted 

 instantaneously, the agitated air still retaining all its heat of 

 combination. From these remarks it appears that the Boylean 

 law ought not to be adopted in the investigation of the velo- 

 city of sound, without some modification. It is obvious that 

 the elastic force of air, which changes its volume while- it re- 

 tains the whole of its absolute heat, ought to be employed. 

 When this explanation is attended to, the difference be- 

 tween theory and experiment is so small that it may safely be 

 ascribed to unavoidable errors of observation. 



The question now to be determined, is, how far this prin- 

 ciple will affect the velocity of sound. Messrs. Clement and 

 Desormes by experiment have determined the constant*, which 



we 



* This quantity is said to be constant; but as its determination by very 

 accurate experimenters varies considerably, it may, it is conjectured, un- 

 dergo a slight modification according to circumstances. The French aca- 

 demicians 



