[ 101 ] 



XL Reply to some Remarks'^ of the Hon. J. W. Strutt on Ga- 

 seous Pressure. By Robert Moon, M.A.j Honorary Fellow 

 of Queen's College, Cambridge f, 



IN criticising my doctrine that density alone does not deter- 

 mine pressure but velocity also requires to be taken into 

 account, Mr. Strutt evidently thinks that he has placed me in a 

 dilemma by asking, ''^ What is here meant by velocity ?^^ Is it 

 " the absolute velocity which is intended," or does it mean " the 

 velocity relatively to the containing vessel?" 



To this I reply, certainly not the velocity relatively to the 

 containing vessel, which, since "no account is taken of friction 

 or viscosity," may, as Mr. Strutt has pointed out, have any ar- 

 bitrary motion in a direction parallel to its axis without affecting 

 the motion of the air within it. 



But if I am asked whether the velocity to which I refer is in- 

 clusive or exclusive of the velocity which the aerial particles 

 whose motion is being considered have in common with the con- 

 tiguous particles of the earth^s surface, I answer that it is a 

 matter of indifference whether we do or do not include that 

 common velocity. 



If the expression which I have given for the pressure in terms 

 of the velocity and density be referred to, it will be found to 

 contain an arbitrary function, the form of which in any parti- 

 cular case of motion must be determined by a consideration of 

 the relations existing between the pressure, density, and velocity 

 under particular circumstances. The form of the function when 

 the absolute velocity is dealt with will be different from that 

 which occurs when the relative velocity is made use of; but the 

 expression for the pressure in either case will be precisely the 

 same. 



I am unable to understand the import of the question, ''Is it 

 not obvious that the physical condition of a small mass of air is 

 independent of any velocity animating all its parts ?" 



If a variable velocity affecting the different particles is here 

 referred to, I answer in the negative ; but if a uniform velocity 

 common to all the particles be meant, I agree with my critic, at 

 the same time regretting that he should not have taken more 

 pains to ascertain my views before ascribing to me an opinion 

 which would be simply preposterous. 



Though Mr. Strutt is quite certain that my "analytical argu- 

 ment .... is fallacious," I am inclined to think that, upon re- 

 flection, he will change his mind upon the subject. 



* See Phil. Mag. S. 4. vol. xliv. p. 64, 

 t Conimimicated b)^ the Autnur. 



