1 86 Prof. Forbes's Researches on the Vibrations which take place 



has been sometimes the case, I used a block of hard metal, 



Fi C . 8. 



with an angle much more acute than that shown at e, and 

 placed a bar of lead upon it, the effect was less favourable 

 than when the angle was more obtuse, and the contact might 

 seem to be greater. The truth, however, was, that in this 

 case the lead, from its softness, was cut by the harder metal, 

 and a new adhesion produced, as in the action of a wedge. 



4-3. These and many other experiments have proved to 

 me, that, to facilitate the vibrations as much as possible, we 

 must have a minimum of adhesion; thus their frequency will 

 be increased and the note raised. Mr. Trevelyan states, that 

 if the surfaces in contact of the two metals be highly polished, 

 no vibration will ensue: this manifestly depends upon the 

 same principle, the adhesion between two perfect planes being 

 well known to be great in amount. I have not met with so 

 strong a case in the course of my experiments. 



44. We may conclude this head by noticing that the inter- 

 ference of any foreign matter between the metals (with the 

 exception of the metallic pellicles already mentioned) seems 

 fatal to the experiment. Dust, amalgam, a coating of oxide, 

 or even oil-gilding, stops the vibration. The action of mer- 

 cury is probably by increasing the adhesion. 



3. Influence of Temperature. 



45. We have seen that the metal of greatest conducting 

 power must have the highest temperature in the combination 

 of two required to produce a vibratory motion. Not merely 

 is there no action between two metals when the temperature 

 of both is the same with air of an apartment, but likewise when 

 both are raised to any higher temperature, for example, that 

 of boiling water. 



46. 1 have not ascertained what is the smallest difference 

 of temperature requisite to produce the effect. It varies, of 

 course, with every different pair of metals. With lead and 

 copper, for example, the vibrations will continue much longer 

 than with lead and tin, although in the former case the tem- 

 peratures tend more rapidly to an equilibrium. 



47. A difference of temperature of 150° seems to be suffi- 

 cient for all practical purposes. Being .anxious to investigate 



