Mr. R. Moon on the Law of Gaseous Pressure. 101 



(where m=x» + -), which, as Mr. Strutt appears to be fully con- 

 scious, satisfy the equation of motion 



°=Sr+il* w 



In his first paper (see Phil. Mag. for July last) Mr. Strutt ap- 

 peared to regard Boyle's law as experimentally established in all 

 cases of rest or motion — a circumstance which, if it were true, 

 would be decisive as to any value which the above formulas may 

 possess in physics. Inasmuch, however, as after my pointing out 

 that the proof of Boyle's law was limited to the case of equilibrium, 

 Mr. Strutt does not repeat his statement, I might conclude that 

 he had altered his opinion, but for the occurrence of certain ex- 

 pressions of a contrary tendency. 



I desire to know definitely, therefore, first, whether Mr. Strutt 

 still considers that Boyle's law has been experimentally proved 

 in the case of motion, and, secondly, what are the experiments 

 upon which he rests this conclusion. 



From not having bestowed sufficient consideration on the for- 

 mulae, Mr. Strutt has completely misapprehended their signifi- 

 cance. 



For, suppose that, when t — Q,p and v have respectively the 

 definite values /, (a?), / 2 (#) 5 then (1) gives us 



p= -w, + *{ Mx)+ m}-' • • • (3) 



but this does not determine the law of pressure which prevails 

 when £=0, for the obvious reason that is arbitrary. So far 

 is it from being the fact that a knowledge of the law of density 

 and the law of velocity prevailing in the fluid at a given time 



* Mr. Strutt, after writing the equation 

 dv _ 1 dp 

 dt D dx' 

 properly remarks that another equation requires to be added, viz. 



d_ /D\ = efo 

 dt\ol dx 



This last, however, is included in the single equation of the text, in virtue 



dx p' 



d (dy\_ d jdy\ dy __D 



of the analytical condition,-^: Ijw~~ fa\di)> anf * of the fact that — 



