ON THE MATHEMATICAL THEORY OF FLUIDS. 247 



latent heat and of specific heats, are employed in solving the 

 problem of the velocity of sound, the solution of which, as vras 

 before remarked, Poisson has derived from the fundamental 

 properties of elastic fluids considered as data of observation. 



With reference to the preceding theory it may be remarked, 

 that although it conducts by simple analysis to the fundamental 

 properties of elastic fluids, and would seem on that account to 

 possess the character of truth, yet it does not appear to have 

 been very generally received, and by some is considered to be 

 not sufficiently natural. I will venture to suggest a reason for 

 this, which is equally applicable to some other of the more abs- 

 tract physical theories, viz. that after we have gone through 

 the mathematical reasoning, and been satisfied of its correctness, 

 on recurring to the original hypotheses, there is some difficulty 

 in judging of them or comprehending them by comparison with 

 anything we see or know by experience. They are too little 

 analogous to facts of observation. If a theory cannot rest on 

 experimental facts, it ought at least to contain no hypotheses 

 which may not be distinctly understood from our experimental 

 knowledge : possibly it is not otherwise a view of the real facts 

 of nature. In short, the evidence for the truth of hypotheses 

 which from their nature do not admit of immediate verification 

 by experiment, must depend as much on the facility with which 

 they are conceived in the mind, and can be expressed in terms 

 of acknowledged import, as upon the accordance of the mathe- 

 matical results they lead to with experience- 

 In one* of the volumes of the Journal of the Polytechnic 

 School there is an elaborate memoir by M. Poisson, which com- 

 prises the substance of two preceding memoirs on the equili- 

 brium and motion of elastic bodies, and on the equilibrium of 

 fluids, and concludes with calculating, according to the principles 

 of the reasoning contained in the preceding part, the pressure of 

 fluids in motion. Throughout this work the reasoning is con- 

 ducted on the hypothesis that bodies are formed of disjoined 

 molecules, separated fi*om one another by spaces void of ponder- 

 able matter, which is considered to be " actually the case in 

 nature;" and the chief object in view is to form the general 

 equations of the equilibrium and motion both of elastic bodies 

 and of liquid and aeriform bodies, according to this hypo- 

 thesis, in a manner as simple and as free from difficulty as 

 possible. By taking account of the void spaces separating 

 the atoms, it is found that the pressure is expressed, not by 

 an integral, but by a sum, which, on the supposition that the 

 intervals between th« molecules are small compared to their 

 * Tom. xiii. call, 20, p. I. 



