408 Prof. Potter's Reply iu the Objeciiuns advanced against 



I may be permitted to add, that Professor Potter's determi- 

 nation of the velocity of sound is also hypothetical. Supposing 

 his method to api)ly to a continuous fluid, such as the air is con- 

 sidered to be when treated hydi'odynamically, it amounts to assu- 

 ming, that, together with a motion of the fluid along a fixed 

 axis, there is a motion transverse to that axis. By a certain rela- 

 tion between these two motions, an elementary portion of the 

 fluid situated on the axis may remain of a cubical form in two 

 successive instants, while it changes its size. But an unlimited 

 number of such relations may be arbitrarily assumed, each of 

 which corresponds to a certain change of form of the element 

 and a certain velocity of propagation. How, then, is that par- 

 ticxdar relation to be selected which gives the actual velocity of 

 propagation ? The hydrodyuamical researches of which I have 

 spoken abo\c, conduct to the general result, that vibratory motion 

 of a compressible fluid, so far as it is independent of any arbi- 

 trary disturbance, consists of motion partly along, or jiavallel to, 

 an axis, and partly transverse to the axis, and determine a rela- 

 tion between the two motions, such that the resulting velocity 

 of sound is that of which I have already given the expression. 

 I am. Gentlemen, 



Your obedient Servant, 



Cambridge Observatory, J. Challis, 



April 17, 1851. 



LIII. A Reply to the Objections advanced against the paper on 

 Sound, in the February Number. By Professor Potter, A.M.'^ 



I WAS much surprised to find so complete a niisappi-ehension, 

 of what is meant by the atomic constitution of bodies, could 

 be entertained by the Rev. Mr. Haughton, as is shown in his 

 paper in the last Number of the Magazine. 



The contraction of bodies in volume under the greatest de- 

 grees of cold which are known, shows that the ponderable parts 

 of bodies cannot be considered to be in contact, but must be 

 surrounded Ijy imponderable atmospheres. The only philoso- 

 phical view which can be taken of tlie actual state of bodies is 

 that of equilibrium under the action of opposing forces. In 

 solids and liquids we have the attractions of the ponderable 

 atoms for each other balanced by the repulsion of the caloric, 

 which, though not affecting its mass or weight, must be consi- 

 dered as an essential portion of the body in its actual state. In 

 gases, the repulsive force due to the caloric requires an external 

 pressure in order to produce equilibrium. We hence have no 

 means of judging of the actual form of the ponderable part, 

 mass, or nucleus of au ultimate atom, as Mr. Haughton seems 

 * Communicated bv tbc Author. 



