206 M. Savart on the Acoustic Figures 



Hence the following rule in words at length. 



To find the thickness of metal — Multiply the pressure per 

 square inch by the radius of the cylinder, and divide the pro- 

 duct by the difference between the cohesive power of the me- 

 tal per square inch and the pressure per square inch, and the 

 quotient will be the thickness sought. 



As an example, let it be required to determine the thickness 

 of metal in two presses, each 12 inches in diameter, in one of 

 which the pressure is \\ ton, and in the other 3 tons, per cir- 

 cular inch. The cohesive force of cast iron being 18,000 lbs. 

 per square inch. ^ 



Here 1| ton per circular inch =4,278 lbs. per square inch. 

 8 tons ditto =8,556 lbs. ditto. 



Whence by the rule 



4,278x6 /-';. * ... 



18,000-4,278 = 1-87 incheS thlckness > 



, 8,556x6 

 and - , ,. _.- — ^.,. =5-43 inches thickness. 

 18,000 — 8,5o6 



Whereas on the usual principle of computation, the one of 



these thicknesses would be exactly double the other. 



Art. XXI.— On the Acoustic Figures produced bjj the Vibra- 

 tions communicated through the Air to Elastic Membranes. * 

 By M. Felix Savart. 



In order to perform the experiments described by M. Savart, 

 we must stretch a thin sheet of paper, about four or five inches 

 in diameter, over the mouth of a vessel, such as a large glass 

 with a foot-stalk, so that the paper has an uniform degree of 

 tension, and a horizontal position. A thin layer of fine and 

 dry sand being then scattered over the paper, a plate of glass, 

 in a state of vibration, is brought within a few inches of the 

 membrane. The vibrations of the glass plate are conveyed 



" This article is a brief abstract of an elaborate paper by M. Savart, en- 

 titled Recherches sur les Usages de la Membrane du Tympan, el de I'oreille 

 extcrne, read to the Academy of Sciences on the 29th April 1822, and 

 printed in the Ann. de Chim. torn, xxvi- p. 1. 



