22 Prof. Forbes's Researches on the Vibrations lohich take place 



intensity and distinctness of vibrating power. In this inquiry 

 I found many difficulties, chiefly arising from the apparently 

 capricious nature of the effects produced, which for a long 

 time seemed almost to baffle an attempt at classification ; and 

 it was only by reiterated series of experiments, at different 

 times, and made in different ways, that I could satisfy myself 

 of the degree of accuracy to which my results were entitled. 



18. I soon found that the conditions of vibration depended 

 simply on the distance between the places of the two metals 

 employed for the bar or block, in a certain scale required to 

 be determined. The remarkable case of iron already observed 

 by Mr. Faraday made me very desirous to extend such a law; 

 here we have a metal which must be placed towards the mid- 

 dle of such a scale, since a rr.qtal above it in the scale vibrates 

 upon it when hotter fhan the iron, whilst iron itself vibrates 

 upon cold lead, which, therefore, must be placed lower in the 

 scale ; and as the intensity of vibration may be expected to 

 be proportional to the interval in the scale, so we actually find 

 that silver vibrates on lead much more actively and steadily 

 than iron does. 



19. I first prepared bars similar to those which have been 

 already described, of copper, zinc, brass, iron, tin, lead, anti- 

 mony and bismuth. My earliest experiments demonstrated 

 the small number of pairs of metals between which vibrations 

 took place. The superiority of lead to all other metals, as 

 the cold substance, was manifest; and in order to establish, 

 in relation to it, the intensity of vibration of the different 

 heated metals, it was necessary to obtain some ready means 

 of employing them all at a fixed temperature. Without en- 

 tering minutely into the actual difference of temperature be- 

 tween the two metals requisite for producing 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 conclude that a differ- 

 ence of 150° 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. 

 j Iron ; decidedly less than brass, 

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

 difference is inconsiderable. 

 Antimony; not at all. 

 Bismuth ; not at all. 



