432 Prof. FORBES on the Vibrations which take place 



the corresponding 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 complete- 

 ly established the dependence of these sounds upon the vi- 

 brations. He did not, however, include in his explanation the 

 action of the groove in producing them, which therefore be- 

 came an object of my attention. To shew 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 impinging 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. In order to shew that the motion of air had nothing 

 whatever to do even with 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 repeated with the same 

 results. We must, therefore, in its fullest extent, deny the in- 

 fluence 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 

 phenomena essentially depend upon the form of apparatus em- 



* Lecture at the Royal Institution April 1831, and Royal Institution Journal, 



No. IV. N. S. 



