1 88 Prof. Forbes's Researches o?i the Vibrations which take place 



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 

 conspicuous phaenomena, was pretty generally acquiesced in, 

 and seems to have acted as a barrier to further examination. 

 It was, I believe, first thrown out by Sir John Leslie, on con- 

 sidering the simple facts which were brought to light by Mr. 

 Trevelyan's experiments, 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 im- 

 pulse for sustaining a new vibration. Even at first sight it 

 does appear very difficult to conceive how, when the vibra- 

 tions are increased to 500 or more in a second, a process de- 

 pending upon so slow an operation 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 instantaneous 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. 



52. This more difficult task was undertaken by Mr. Fara- 

 day; and in a lecture on the subject which I was fortunate 

 enough to hear at the Royal Institution in April 1831, he 

 freed the subject, (as we have already seen,) from many of the 

 difficulties with which it had been surrounded, and illustrated 

 the theory which he supported in that happy style for which 

 he is so remarkable. 



53. The principle which he adopted was fundamentally the 

 same as that of Sir John Leslie, but he added an explanation 

 of the influence of the properties of different metals upon the 

 phaenomena. According to his view, the hot metals should 

 have a higher conducting power, and a smaller expansion by 

 heat, than the cold one, and the arrangements of the metals 

 as vibrators depend, according to him, upon this principle. 

 To employ the official statement of his views contained in the 

 Royal Institution Journal *, " the superiority of lead, as the 

 cold metal, was referred to its great expansive force by heat, 

 combined with its deficient conducting power, which is not 

 a fifth of that of copper, silver, or gold; so that the heat ac- 



* New Series, No. IV. pp. 119, 122. 



