10 Dr. Tyndall on the Vibrations and Tones produced by 



rocker to rest upon a flat surface, while the iustrument itself 

 leans slantingly against the bearer. In delicate experiments, I 

 think a knife-edge is a better support for the handle, the rocker 

 . being placed horizontal, or nearly so. 



Omitting the last three substances, which might, perhaps, 

 with some justice be regarded as metallic, we find a number of 

 exceptions to the law under consideration, which far exceeds the 

 number of bodies mentioned in the paper of Professor Forbes. 

 These exceptions demonstrate that the second law also is untenable. 



The third general law runs as follows : — 



" The vibrations take place with an intensity proportional {within 

 certain limits) to the difference of the conducting powers of the 

 metals for heat, the metal having least conducting power being 

 necessarily the coldest." 



The evidence adduced against the validity of the first law ap- 

 pears to destroy this one also ; for if the vibrations are to be 

 ascribed to a difierence in the conducting powers of the rocker 

 and bearer, then when there is no such difierence there ought to 

 be no vibrations. But we have shown, in half a dozen cases, 

 that vibrations occur when rocker and bearer are of the same 

 metal. The same facts deprive the latter part of the third law 

 of its significance. 



I will however cite one or two experiments, in which the con- 

 ditions regarded necessary by Professor Forbes were reversed, 

 and the effect was produced notwithstanding. 



1. Silver stands at the head of the conductors of heat. A 

 copper rocker was laid upon the edge of a thin plate of this 

 metal ; strong musical notes were obtained from the arrangement. 



2. Forcible vibrations were produced by placing a brass rocker 

 upon the same silver plate. 



3. A feeble, but distinct tone, was produced by the iron 

 rocker. 



4. Gold is a better conductor than brass ; nevertheless strong 

 vibrations were obtained by placing a hot brass rocker upon the 

 edge of a half-sovereign. 



These experiments are, I think, sufficient to prove the non- 

 existence of the third law. 



In the prosecution of his inquiry. Professor Forbes discovered 

 *' that at leasjb two metals were perfectly inert in either situation, 

 namely, antimony and bismuth.^' Considering the explanation 

 given, that the effects are due to the mechanical repulsion exerted 

 by the heat in its passage from a good conductor to a bad one, 

 the inertness of the two bodies mentioned presents a grave diffi- 

 culty. Reflecting on the subject, the thought occurred to me, 

 that if a mass of bismuth or antimony were cut so that the plane 

 of moat eminent cleavage might be vertical, the superior con- 



