ticj/l*-*^ K U 



ON THE PROPAGATION OF SOUND. 371 



may be completely reconciled with experiments; we may estimate the mo- 

 dulus of the air's effective elasticity,which is the measure of its immediate force, 

 from the velocity which is thus observed, and its height will appear to be 

 ZQ 800 feet, instead of 27 800, which is the supposed height of the atmo- 

 sphere. This velocity remains unchanged by any alternation of pressure in-, 

 dicated by the barometer, but it may be aftected by a change of temperature, - ; ^ 

 For when an elastic fluid is compressed, its elasticity is increased in the same 

 ratio as its density; and the height of a homogeneous atmosphere equiva- 

 lent to the pressure, remains the same, consequently the velocity calculated 

 from that height remains unaltered; but the identity of the acceleration, 

 from the effect of heat which has been mentioned, can only be inferred 

 from observation : this identity may, however, be satisfactorily shown, by 

 means of experiments on the sounds of organ pipes, whicli are intimately 

 connected with the velocitv of the transmission of sound through the air, 

 and which are found to remain precisely the same, however the air may be 

 rarefied or condensed. . The Academicians del Cimento inclosed an organ 

 pipe, with bellows worked by a spring, in the receiver of an air pump and 

 of a condenser, and they found-" that, as long as the sound was audible, 

 its pitch remained unchanged. Papin screwed a whistle on the orifice 

 winch admits the air into the receiver of the air pump, and 1 have fixed an 

 organ pipe in the same manner; and the result agreed with the experiment, 

 of the academicians. But if the density of the air is changed, while its elas- 

 ticity remains unaltered, which happens when it is expanded by heat, or con- 

 densed by cold, the height of the column, and consequently the velocity, 

 will also be altered; so that for each degree of l-'ahrenheit's thermometer the 

 velocity will vary about one part in a thousand. Bianconi has actually ob- 

 served this difference of velocity according to the different heights of the 

 thermometer, and it may be shown less directly by means of the sounds of 

 pipes; but it has not been accurately determined. whether or no the correc- 

 tion on account of the effect of compression in causing heat, remains unal- 

 tered, although Bianconi's experiments agree very well with the supposition 

 tliat no material change takes place in this respect. The velocity of sound 

 must also be in some measure influenced by the quantity of moisture con- 

 tained in the atmosphere : it must be a little diminished by cold fogs, which 

 add to the density, without augmenting the elasticity, and increased by 

 warm vapours, which tend to make the air lighter; and these two opposite 



