392 



by the author from experiments by Messrs. Thomson and Joule, 

 Tq=272^° Centigrade = 490^° Fahrenheit, a value which may be 

 considered sufficiently correct for practical purposes. 



The maximum theoretical efficiency of every conceivable thermo- 

 dynamic engine receiving heat at the temperature Tj, and giving out 

 heat at the temperature To, is 



Qi-Q^ - T.-T, 

 Qi T, + To' 



The fourth section concludes with a system of formulae, illustrated 

 by numerical examples, for computing the power ond efficiency of 

 air-engines. 



In the fifth section, the principles of the preceding sections are 

 applied to aggregates consisting of heterogeneous substances, or of 

 the same substance in different conditions, especially the aggregate 

 of a liquid and its vapour ; and the results are applied to the nume- 

 rical computation of the theoretical efficiency of steam-engines. 



Jan. 26. 1854. 

 The REV. BADEN POWELL, V.P., in the Chair. 



1. A paper was read, entitled *' On the Vibrations and Tones pro- 

 duced bv the contact of bodies having different Temperatures." By 

 J. Tynd'all, Esq., F.R.S. Received Jan. 15, 1854. 



The author introduces the subject of his paper by a brief descrip- 

 tion of the labours which have preceded his own, from the discovery 

 of the phsenomenon by Schwartz in 1805 to its revival and further 

 examination by Trevelyan, Faraday, Forbes and Seebeck. The pe- 

 culiar views of Prof. Forbes, who regards the effects as due to a new 

 species of mechanical agency in heat, were the chief inducement to 

 the resumption of the subject by the author. He examines the 

 ground on which the theory of Prof. Forbes is based, and tests by 

 experiment the general laws at which he has arrived. The first of 

 these laws is, that " the vibrations never take place between sub- 

 stances of the same nature." By converting the cold metal on which 

 the hot rocker is placed into a thin plate, fixing this plate in a vice, 

 and causing the rocker to rest upon the edge of the plate, the author 

 obtained vibrations with iron on iron, brass on brass, copper on copper, 

 silver on silver, zinc on zinc, tin on tin ; and thus shows the first 

 law to be untenable. The second general law affirms that " both 

 substances must be metallic." As exceptions to this law, the author 

 adduces experiments made on about twenty non-metallic substances, 

 with which perfectly distinct vibrations were obtained. Among those 

 which signalize themselves by the force and permanence of the 

 rockings they produce, are to be found rock-salt, fluor-spar and 

 rock- crystal. With rockers similar to those described in the paper, 

 and attending to the precautions there dwelt upon, vibrations and 

 musical tones can be obtained without difficulty on these substances. 



