Fig. 6. 



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



in neither case is the connexion of these points in any way 

 essential. 



39. The use of bars made entirely of the different metals is 

 therefore quite unnecessary. A convenient form of experi- 

 mental apparatus is suggested by the following construction, 

 which I have employed with success. A heated ring of brass 

 or copper, three or four inches in diameter, bein<r placed side- 

 ways upon a ridge of solid lead, with two solid angles, upon 

 which the ring may vibrate (the plane of the ring being hori- 

 zontal), the action will be extremely energetic, the impulse 

 being given simultaneously at two points, as shown in rig. 6. 

 If we had the means of firmlv 

 clamping two slips of any metal 

 under experiment to the two 

 points of the ring in contact 

 with the block, by means of 

 tightening screws, so as to sub- 

 stitute the material required for 

 that of which the ring is made, 



we should have a convenient apparatus, requiring very small 

 pieces of the metals to be tried, and therefore well adapted 

 for experiments on gold, silver, &c. 



40. The influence of the thickness of the metals employed, 

 and of the extent through which the impulse may be given, 

 early attracted my notice ; and I found that thin films of me- 

 tals of superior conducting power, in the form of leaf, burn- 

 ished upon the lead block, did not annihilate its characteristic 

 property. The same result in regard to simple gilding was 

 announced by Mr. Trevelyan. 



41. We have now to resume the consideration of an im- 

 portant point connected with figure, referred to in an early 

 part of this paper ; I mean the groove in the bar or block, 

 which frequently appears essential to the production of a 

 musical note. We have already dismissed the supposition 

 that it has any connexion with the passage of air through 

 that groove, and referred the effect solely to the actually ob- 

 served increase of velocity in the oscillations : it still remains 

 to explain this result. After a very careful consideration of 

 the phenomena, I am disposed to account for it entirely upon 

 the diminution of the surfaces in contact. It may at first 

 sight be thought that the adhesion of two metallic surfaces 

 must be too small to influence sensibly the time of an oscilla- 

 tion : when the enormous velocity of these oscillations is, how- 

 ever, considered, there can be no room for astonishment. We 

 have shown that there are frequently more than Jive hundred 

 contacts and separations in a single second. The most minute 



