between Metallic Masses having different Temperatures. 19 



thus lengthening the period of its vibrations ; and Mr. Fara- 

 day well observed, that the pressure of the finger upon the 

 upper part of the bar, while in a state of vibration, by short- 

 ening the period of oscillation, raised the note to a great ex- 

 tent, and rendered the sounds musical, when before they were 

 not so. In the bars which I have generally employed, the 

 back was hollowed out, as shown in the section fig. 4, which 

 served both to diminish the quantity Y\<t. 4. 



of matter near the axis, and to con- 

 tain a portion of mercury, the bright 

 surface of which 1 found a convenient 

 test of the existence of oscillations too 

 minute to be readily perceived by the eye, as was o-enerally 

 the case when the sounds were musical. They could always 

 be detected likewise by approaching the point of the nWer 

 gently to the bar. This will even detect vibrations which pro- 

 duce no visible tremor in a clear globule of mercury. 



6. I trust I have now shown that all the phenomena of 

 sound are ultimately resolvable into those of simple vibration. 

 There are one or two facts connected with, and indicated by, 

 the musical tone, which we shall more particularly consider 

 in treating of the modification of the vibrations. These are 

 especially the sudden changes in the note of the instrument, 

 which generally rises as the experiment proceeds, and the in- 

 fluence of the groove in raising the note, that is, rendering 

 the vibrations more frequent. 



7. We have only a remark to make upon the absolute velo- 

 city of these oscillations, which the sound produced affords 

 the only accurate way of estimating. With one of the ordinary 

 bars, such as I have been in the habit of using, the highest 

 note which I have distinctly observed and compared, was A 

 above the middle C of the pianoforte, which corresponds to 

 430 vibrations in a second. From this velocity (and it is 

 often very much greater,) the tone descends through all the 

 lower notes down to the smallest number of vibrations pro- 

 ducing 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 pheno- 

 mena of vibration, and we must seek an explanation of the 

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



II. Phenomena of Vibration. 



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

 is put in motion upon a block of metal, the temperatures be- 

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

 the simple action of gravity are very speedily annihilated*. 



D2 



