434 Prof. FORBES on the Vibrations which take place 



greater), the tone descends through all the lower notes down to 

 the smallest number of vibrations producing a musical note, even 

 to about 20 in a second. 



8. From what has been now said, we are prepared to main- 

 tain that the phenomena of sound are all referable to phenomena 

 of vibration, and we must seek an explanation of the modifica- 

 tions observable in the former, in those of the latter. 



II. PHENOMENA OP VIBRATION. 



9. When a rocking bar, such as has been above described, is 

 put in motion upon a block of metal, the temperatures being 

 equal, the vibrations gradually diminish in extent, and by the 

 simple action of gravity are very speedily annihilated. That this 

 may not be the case, as we find it is not under some circumstances 

 already alluded to, namely, where certain different metals, and at 

 different temperatures, are employed, we must admit the exist- 

 ence of some impulse which prolongs the time during which the 

 oscillations are kept up. This impulse can only be received du- 

 ring the successive contacts at the two bearing points of hetero- 

 geneous metals, and may safely be assumed to depend in some 

 way or other upon the propagation of heat, since the effect does 

 not take place, unless the temperatures be different, nor is it in- 

 different which of the two kinds of metal has the highest tem- 

 perature. The impulse, of whatever kind it be, resembles that 

 derived by a pendulum from the pallets of a timekeeper, which 

 in fact is the sustaining power of the mechanism. 



10. The arcs of vibration of course depend, other things be- 

 ing equal, upon the intensity of the impulse communicated to 

 the bar. 



11. We have already noticed, that various circumstances 

 tend to modify materially the character of the vibrations, parti- 



