1823.] Mr. Wheat stone on Sound: 83 



in the uniform parts. I afterwards inserted this glass In anothef 

 vessel of water in order to observe the vibrations of the external 

 surface, and found the same results as in the interior, though the 

 levels of the surfaces were different. '; 



The most accurate method to observe these phenomena is by- 

 employing a metallic plate of small dimensions, which must be 

 fixed horizontally in a vice at one end, and covered on its upper 

 side with a surface of water : on causing it to oscillate entirely 

 by means of a bow, a regular succession of these vibrating cor- 

 puscles will appear arranged parallel to the two directions of the 

 plate, and if the action of the bow be rendered continuous, their 

 absolute number might be counted with the aid of a micrometer. 

 Diminishing the oscillating part of the plate to one half of its 

 length, the double octave to the preceding was heard, agreeably 

 to the established rule, that the velocities of the oscillations are 

 inversely as the squares of the lengths ; four vibrating corpuscles 

 then occupied the space before occupied by one, and the abso- 

 lute number was double to that in the former instance ; but the 

 absolute number of these corpuscles have no influence whatever 

 on the degree of tune, which entirely depends on their relative 

 magnitude in the same substance ; theory shows us that in 

 plates of this description alteration of breadth does not affect the 

 degree of tune ; let us, therefore, reduce this half of the plate to 

 half its breadth, and we shall find the note remain the same, but 

 the absolute number of the corpuscles will in this case be equal 

 to that in the entire plate. Let us now take two plates of equal 

 lengths and breadths, but one double in thickness to the other; 

 the rule is, that the velocities of the oscillations are as the thick- 

 nesses of the plates ; we shall, therefore, in the thicker plate see 

 a double number of particles to that of the other, occupying the 

 same extent of surface. The last circumstance in which two 

 plates may differ is their specific rigidity, and in this respect it 

 will be found that two plates of exactly equal dimensions, and 

 covered with the same number of vibrating corpuscles of equal 

 magnitudes, but of different substances, differ in sound ; there- 

 fore, the absolute magnitudes of the particles cannot be assumed 

 as a standard of tune, unless regulated by the specific rigidity. 



Unassisted by any means of actual admeasurement, the above 

 are but the proximate results sensible to the eye; more extended 

 and accurate experiments are necessary to confirm the results 

 with mathematical certainty. As the absolute magnitudes of 

 these particles will, I imagine, be hereafter a most useful element 

 for calculation, I wall here indicate the most effectual way I am 

 acquainted with to arrive at this knowledge. A thick metallic 

 shp of considerable length and breadth, bent similarly to a tun- 

 ing fork, and fixed at its curved part in a vice, is very easily 

 excited by friction, and a more considerable surface of regularly 

 arranged vibrating particles is seen than in most other superfi- 

 cies; any description of common exciter may be employed, 



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