296 Mi*. A. S. Davis on the Vibrations which Heated 



position was also observed. The conducting-power of the 

 secondary circuit was =119, and the intensity of the inducing 

 current = tan 28°. The following deflections were observed. 

 The signs denote, as above, the side of the scale towards which 

 the magnetic needle was deflected. 



The primary circuit broken. The secondary coil raised. 



-360 -36*0 



35-8 36-8 



360 36-4 



36-2 36-8 



Mean —36'0 Mean -36'5 



The circuit closed. The coil lowered. 



+ 36-2 +36-0 



36-4 36-2 



36-4 35-4 



Mean +36-0 Mean +35"9 



Thus, according to the theory, the intensity of the secondary 

 current is the same for all four ways of effecting the induction. 



XXXVIII. The Vibrations which Heated Metals undergo when in 

 contact with cold Material, treated mathematically. By A. 

 S. Davis, M.A.* 



UNDER certain conditions a heated piece of metal, when 

 laid upon a cold block of metal or some other suitable 

 material, will vibrate by rocking rapidly about upon its points of 

 support ; and the vibrations will continue as long as the heated 

 metal continues much hotter than the cold block upon which 

 it rests. 



One of the conditions essential for the production of these 

 vibrations is that the heated metal must be so shaped and so 

 placed upon the cold block that it would, if cold, rock rapidly to 

 and fro for a short time upon being slightly tilted and left to 

 itself. This condition may be attained by shaping the piece of 

 metal with two parallel ridges near together, on which it may 

 rest upon the cold block, whilst it finds a third point of support 

 on the table upon which the block is placed. The heated piece 

 of metal may consist of brass, copper, iron, or generally of any 

 of those metals which are good conductors of heat. The best 

 materials known for the cold block are lead and rock-salt. 



Sir John Leslie first suggested that the cause of these vibra- 

 tions is to be found in the expansion of the cold block by the 

 heat which flows into it from the hot metal at the points of con- 



* Communicated by the Author. 



