between Metallic Masses having different Temperatures. 453 



on platinum when at a moderate temperature, than when red 

 hot. The effect may, however, be connected with the theory of 

 the vibration. 



50. Having now discussed the phenomena of sound, and of 

 the vibrations to which we have shewn these sounds to be re- 

 ferrible, we shall next consider 



III. THE THEORY OF THE PHENOMENA. 



51. IT is a curious fact how imperfectly the interest attached 

 to the phenomena observed by Mr TREVELYAN, seems to have 

 excited enlightened curiosity. Indeed, an explanation of great 

 simplicity, and which appeared to account for the more con- 

 spicuous phenomena, was pretty generally acquiesced in, and seems 

 to have acted as a barrier to farther examination. It was, I believe, 

 first thrown out by Sir JOHN LESLIE, on considering the simple 

 facts which were brought to light by Mr TREVELYAN'S experi- 

 ments, that they might be explained by the expansion of the 

 cold metal at the instant of contact with the warm one, which 

 might be supposed to give a sufficient impulse for sustaining a 

 new vibration. Even at first sight it does appear very difficult 

 to conceive how, when the vibrations are increased to 500 or 

 more in a second, a process depending upon so slow an opera- 

 tion as the conduction of heat, should cause the metal to expand 

 and contract successively by a finite quantity. The effect has 

 every appearance of being one of active and almost instantane- 

 ous repulsion, and bears no resemblance whatever to the slow 

 mechanical elevation of the surface by the process of expansion. 

 But such inferences are often erroneous ; it became, therefore, 

 most important to inquire how far the hypothesis was applicable 

 to various forms of the experiment, particularly to the different 

 properties in this respect, of various substances. 



