on the Vibrations of Solid Bodies. 51 



rod, the disc will execute normal vibrations, and it will pre- 

 sent, when it sounds in the air, a circular nodal line, which 

 will cut each of its radii nearly in the middle of its length ; 

 but in water this line will be transferred towards the edge of 

 the disc, and it will approach it in proportion to the difference 

 between the sounds of the body in air and in water. An ana- 

 logous phenomenon occurs in rods: a rod, for example, which 

 presents four nodal lines perpendicular to its edges when it 

 sounds in air, still presents the same number in water, but 

 then such of the lines as were less distant from the extremities 

 of the rod approach to it still more, so that all the vibrating 

 parts are lengthened. We may establish the accuracy of these 

 results by projecting sand through liquids on bodies immersed 

 in them. The nodal lines will be traced as distinctly as in 

 air. 



With regard to the different pressures exerted on the vi- 

 brating bodies, if placed at different depths, we have observed 

 that, when the depth is such that the surface of the fluid re- 

 mains fluid during the vibrations, the sound will continue sen- 

 sibly the same, even to the depth that we can hold it with one 

 hand, while it is made to sound with the other. It ou^ht to 

 be remarked, however, that when the experiment is made in a 

 vessel, we must take care not to allow the vibrating body to 

 approach too near its bottom or sides, because the reaction ■ 

 exerted by these parts, which are then agitated as a system, 

 may alter the number of vibrations, and render the sound 

 more intense. 



I shall conclude this notice with an observation relative to 

 preceding researches on the intensity of sounds propagated in 

 different media. When a body executing normal vibrations 

 sounds in water, for example, then, if we abstract all attendant 

 circumstances, that is to say, if we attend only to the impres- 

 sion made on the ear, we shall conclude that the water transmits 

 the sound with less intensity than air; but if we consider that 

 the mode of vibration in water is no longer the same as in air, 

 that the sound is become more grave, we are compelled to 

 draw the conclusion, that circumstances being no longer the 

 same in both cases, we can deduce no inference respecting the 

 sensation we experience. Hence we cannot consider as exact 



