On the Propagation of Sound between Parallel Walls 301 



We do not, however, intend to speculate further on these 

 results until other specimens of the series have been investi- 

 gated. There are several other very interesting points about 

 this specimen. The hysteresis loss in a complete cycle of 

 the magnetic force is extremely small, as the specimen has a 

 very small coercive force. 



The specimen is very brittle, and when broken is seen to 

 consist of large crystalline masses whose facets have a bright 

 metallic lustre. A careful microscopic investigation of these 

 crystalline masses has not been made; and therefore we are 

 not at present in a position to say whether any obvious change 

 in the formation of the crystalline masses has, or has not, 

 taken place, due to the successive heatings. The appearance 

 of the fracture, however, before and after the heatings is, to a 

 casual observer, very similar. At any rate, we can say that 

 large crystalline masses are present in each case. 



Circumstances have, unfortunately, prevented us from 

 going on with the investigation at the present time, but we 

 trust that it will be possible for one of us to continue the 

 work at a not very distant date. 



January 8, 1901. 



XXV. On a Problem relating to the Propagation of Sound 

 between Parallel Walls. By Lord Rayleigh, F.R.S* 



THE influence of viscosity and heat conduction in modify- 

 ing the propagation of sound in circular tubes of moderate 

 dimensions has been treated by KirchhofFt hi his usual 

 masterly style, but he passes over the case when the diameter 

 is very large. In my book on the ' Theory of Sound/ 2nd 

 edition, § 318, I have given a full statement of KirchhofPs 

 theory, and have indicated the alterations required when the 

 boundary is supposed to take the form of two parallel planes 

 instead of a cylindrical surface. In any case the action of 

 the wall is supposed to be such as to annihilate variation of 

 temperature, and tangential as well as normal motion. In 

 connexion with the problem of the propagation of sound over 

 water I recently had occasion to extend the analysis to the 

 case of a layer of very great thickness, and though, as the 

 result showed, the solution fails to answer the question 

 which I had then in view, it is of some interest in itself. In 

 this case the practical question differs somewhat from that 

 proposed by Kirchhoff, who assumes not only complete perio- 

 dicity with respect to time, but also a quasi-periodicity with 



* Communicated by the Author. 



t Pogg. Ann. vol. cxxxiv. 1868 ,• Collected Memoirs, p. 540. 



