Mr. A. Trevelyan on the Vibration of heated Metals. 327 



not despair but that hereafter, when this subject is better un- 

 derstood, we shall be able to produce vibration with all the 

 metals, and with all other matter either solid or liquid, when 

 heat is applied to a cool substance, or vice versa. 



The following theory, which is partly derived from the late 

 Professor Leslie's mode of accounting for the vibration, and 

 appears the most probable, (and experiments hitherto made 

 strengthen this view,) ascribes the vibratory movements to the 

 usual mechanical changes which caloric occasions in passing 

 from one substance into another, — I mean the expansion and 

 contraction which accompany alternations of temperature. 



It appears that some degree of roughness of one metal is 

 essential to the success of the operation. This slight asperity 

 arises from numberless points or ridges projecting from the 

 mass of metal. 



When the heated bar is laid upon the cool lead, the caloric 

 passes into these prominences; and as their power of conduction 

 is not great, it does not rapidly diffuse itself through the rest 

 of the mass: of course they instantly expand and elongate, and 

 by that sudden elongation they give an impulse to the incum- 

 bent bar. Soon, however, the caloric moves into the adjoining 

 mass, and the prominences contract, and at the same time 

 come into a state ready to admit a renewed accession of ca- 

 loric from the bar : they receive that caloric, again expand, 

 and give a second impulse to the bar. This goes on inces- 

 santly, and though the first impulse be infinitely small, and 

 altogether inadequate to produce any sensible movement of 

 the bar, yet by incessant repetition an accumulation of effect 

 takes place, and the movements gradually reach a sufficient 

 magnitude to become easily discernible. 



As soon as the bar and block arrive within a certain limit 

 of difference of temperature, the impulses become feebler and 

 feebler, and at length the bar comes to rest. It has been 

 mentioned that the smoother the bar is so much greater is 

 the effect: I conceive that this smoothness operates by in- 

 creasing the celerity with which the surface possessed of it 

 communicates the caloric to the projecting points of the block, 

 and thereby the elongation which gives the impulse to the bar 

 is increased both in quickness and extent. 



It is obvious that had the bar any considerable degree of 

 asperity, the points of contact between the two metals would 

 be fewer, and the passage of caloric between them more 

 tardy. 



When the two surfaces are highly polished, the experiment 

 does not succeed : no tremors occur. This result proceeds 

 probably from the circumstance, that the caloric enters into 

 every part of the surface of the block equally, and is more 

 quickly diffused through the mass, and hence there are none 



