438 Prof. FORBES on the Vibrations which take place 



sary to obtain some ready means of employing them all at a fixed 

 temperature. Without entering minutely into the actual dif- 

 ference of temperature between the two metals requisite for pro- 

 ducing the effect, it was sufficient to discover that the heat of 

 boiling water answered every practical purpose ; it was therefore 

 resorted to. The temperature of the lead being 65, we con- 

 clude that a difference of 1 50 between the metals is sufficient 

 to produce the effect in the most decided manner. 



Block at 65". Bars at 212. 



Zinc ; vibrates briskly and steadily. 



Brass ; nearly the same as zinc, but not quite so steady. 



Iron ; decidedly less than brass. 



Tin ; does not vibrate so well as iron, but the difference is 



inconsiderable. 

 Antimony ; not at all. 

 Bismuth ; not at all. 



20. In pursuing these experiments, I varied them in every 

 way I could devise, but almost always got precisely the same ar- 

 rangement of vibrators. Employing a lead bar, I used also 

 blocks of the different metals heated to 212. With silver, gold, 

 and platinum, I found it difficult to procure considerable masses 

 of sufficient purity ; and when small ones taken out of boiling 

 water were employed and placed in a vice, I found that the loss 

 of heat they experienced was so rapid as to destroy the compa- 

 rability of the experiment. The plan I adopted for procuring 

 an approximation to a uniform temperature, and which from re- 

 iterated trials I found susceptible of great accuracy, was the fol- 

 lowing : The piece of metal under experiment being firmly held 

 in a vice, a drop of water was placed upon it. and a spirit-lamp 

 applied below, so as to heat at once the metal and the vice, un- 

 til the water was rapidly dissipated in the act of ebullition, at 

 which instant the cold bar of lead was placed upon it, and the 

 vibrations encouraged by a gentle oscillation. In this manner I 



