458 Prof. FORBES on the Vibrations which take place 



MUNCKE of Heidelberg in Poggendorff's Annalen *, consists 

 chiefly of the translation of a very brief notice, which I had pub- 

 lished in the Edinburgh Journal of Science f, but contains no 

 new observations. Having shewn the reasons which led me to 

 dissent from the opinions at first proposed, I shall now explain 

 the views which I have been led to entertain from the study of 

 the phenomena. 



59. I shall first recapitulate the general laws at which we 

 have arrived. 



1st, The vibrations never take place between substances of 

 the same nature. Art. 13. 



2r/, Both substances must be metallic. Art. 14. 



3d, The vibrations take place with an intensity proportional 

 (within certain limits) to the difference of the conducting powers 

 of the metals for heat (or electricity) : The metal having the 

 least conducting power being necessarily the coldest. Art. 36. 



4>th, The time of contact of two points of the metals must be 

 longer than that of the intermediate portions. Art. 37. 



5th, The impulse is received by a distinct and separate pro- 

 cess at each contact of the bar with the block, and in no case is 

 the metallic connection of the bearing points in the bar, or those 

 of the block, in any way essential. Art. 38. 



6th, The intensity of the vibration is (under certain excep- 

 tions) proportional to the difference of temperature of the metals. 

 Art. 47- 



60. In order to satisfy these various conditions, we shall find 

 that the range of hypotheses is not great. During my experi- 

 ments I was for a long time attached to the idea of a thermo- 

 electric action. The hypotheses which I assumed to explain the 

 steps of it I was forced successively to abandon, and the total 



* 1832, No. III., p. 466. f New Series, No. XI. 



