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LXVI. A Reply to Remarks connected with the Solution of the 

 Problem of Sound. By Professor Potter, A.MJ^ 



PROFESSOR CHALLIS, in the last Number of the Maga- 

 zine, shows how certain hypotheses have been advanced in 

 order to obtain a solution of the problem of sound, and also 

 states what he conceives to be the hj'pothesis on which my so- 

 lution is founded ; from which, however, I dissent. 



Professor Challis misunderstands, I conceive, the effect of in- 

 troducing the atomic theoiy into hydrodynamics when he says, " it 

 amoxmts to assuming, that, together with a motion of the fluid 

 along a fixed axis, there is a motion transverse to that axis/' 

 This relates to my considering the condensations and rarefactions 

 to take place in all directions, which is however implied when- 

 ever Boyle's hydi-ostatical law is used, and no solution of the 

 problem of sound can be attempted without it. As to Boyle's 

 law involving a motion of the atoms sensibly transverse to the 

 wave direction, I do not conceive the view to be tenable. The 

 condensations and rarefactions cannot be considered to arise by 

 the contraction or expansion of definite sheets or shells of atoms, 

 perpendicular to the wave dii-ection ; but by the atoms situated 

 variously on the arrival of a wave, re-arranging themselves in 

 accordance with the varying pressure, and continuing to vibrate 

 so as to fulfill the law. 



Now from the undefined and exceeding minuteness of the 

 ultimate atoms of matter, the lateral motion of any atom may be 

 almost indefinitely small compared with the breadth of a wave, 

 and even with the displacement of the atom from its original 

 position, although this latter is considered small ; and by this 

 intermingling or re-arrangement of the atoms, the requirements 

 of Boyle's law will be fulfilled. There can be nothing like a 

 regular formation of shells or sheets of atoms which do not 

 change their relative positions ; for such sheets of atoms would 

 be incompatible with the extreme fluidity of gases, and the per- 

 petual intestine motion within them, such as is seen by means 

 of the particles of dust in a sun-beam on passing through a still 

 room. The condensations and rarefactions taking place, accord- 

 ing to Professor Challis's method, by a regular transverse motion 

 to or from a fixed axis in such an unstable fluid as air, is evi- 

 dently inconceivable. 



In addition to the received law of Boyle and doctrine of atoms 

 of Dalton, I believe I have proposed no other hypothesis than 

 this — that the equations of motion must be formed for distinct 

 atoms, and the correct methods of forming and employing those 

 equations on the atomic constitution of fluids must be the means of 

 discovering the solutions of hydrodynamicul problems. Whether 



* Communicated by the Author. 

 2K2 



