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



state that the mere accumulation of impulses of any kind up 

 to a certain number in a second, produces alone a musical 

 sound depending upon that number. This Mr. Faraday readily 

 showed to be the true cause of the musical sounds produced 

 in this experiment, namely, the number of contacts of the hot 

 with the cold metal in a second, and he illustrated the fact by 

 putting a cold bar of metal in vibration by means of the cor- 

 responding expansions and contractions of a pair of sugar- 

 tongs connected with it, when sounds quite analogous to those 

 just alluded to were produced*. So far Mr.. Faraday com- 

 pletely established the dependence of these sounds upon the 

 vibrations. He did not, however, include in his explanation 

 the action of the groove in producing them, which therefore 

 became an object of my attention. To show that there was 

 no action such as the impulsion of air through an orifice, 

 which seemed to have been contemplated by Mr. Trevelyan, 

 I caused a hot bar to vibrate upon two pieces of lead, the 

 two striking parts or solid angles of the bar impingeing upon 

 different masses of lead, at a distance of about a quarter of an 

 inch, fig. 3 ; the effect was precisely similar to that of a groove 

 cut of the same breadth in a single piece 

 of lead, and of the smallest possible depth. Fig. 3. 



In order to show that the motion of air _ 

 had nothing what ever to do even with I 

 the tone, I allowed the instrument to \ 

 acquire a steady note with the two, — , — — 

 masses of lead just described, and then ^ 

 carefully closed the edges of the space 

 between the masses with adhesive paste, 

 yet not the slightest change of note was 

 perceptible: this was several times re- 

 peated with the same results. We must therefore, in its fullest 

 extent, deny the influence of any imaginary current of air in 

 the production of the sound. 



5. What, then, is the influence of the groove? The answer 

 is simple, and easily proved by experiment, merely that the 

 rapidity of the vibrations is in some way increased by its 

 presence. When both surfaces are smooth, the vibrations are 

 comparatively slow, and might almost be counted : sometimes, 

 with the form of bar I have used, they do not exceed twenty 

 in a second. In fact, the phasnomena essentially depend upon 

 the form of apparatus employed. Mr. Trevelyan practically 

 found the influence of throwing the mass of matter to the sides 

 of the bar, so as to render its equilibrium more unstable, and 



• Lecture at the Roval Institution April, 1831, and Royal Institution 



Journal, No. IV. N.S. " 



