HEARING. 295 



ble sound, although its parts are visibly thrown into the 

 usual vibratory motions. In proportion as air is admitted 

 into the receiver, the sound becomes more and more dis- 

 tinct; and if, on the other hand, the air be condensed, the 

 sound is louder than when the bell is surrounded by air of 

 the ordinary density.* 



The impulses given by the sounding body to the contigu- 

 ous particles of the elastic medium, are propagated in every 

 direction, from particle to particle, each, in its turn, striking 

 against the next, and communicating to it the whole of its own 

 motion, which is destroyed by the reaction of the particle 

 against which it strikes. Hence, after moving a certain de- 

 finite distance, a distance, indeed, which is incalculably small, 

 each particle returns back to its former situation, and is again 

 ready to receive a second impulse. Each particle, being 

 elastic within a certain range,t suffers a momentary com- 

 pression, and immediately afterwards resumes its former 

 shape: the next particle is, in the mean time, impelled, and 

 undergoes the same succession of changes; and so on, 

 throughout the whole series of particles. Thus, the sono- 

 rous undulations have an analogy with waves, which spread 

 in circles on the surface of water, around any body, which, 

 by its motion, ruffles that surface; only that, instead of 

 merely extending in a horizontal plane, as waves do, the so- 

 norous undulations spread out in all directions, forming, not 

 circles in one plane, but spherical shells; and, whatever be 

 the intensity of the sounds, the velocity with which the un- 

 dulations advance is uniform, as long as they continue in a 

 medium of uniform density. This velocity in air, is, on an 

 average, about 1100 feet in a second, or twelve and a half 



* These facts were first ascertained by Dr, Ilauksbee. See Philosophical 

 Transactions for 1705, vol. xxiv., p. 1902, 1904. 



■|- The particles of water are as elastic, within a limited distance, as those 

 of the most solid body, although, in consequence of their imperfect cohe- 

 sion, or, rather, theii* perfect mobility in all directions, this property cannot 

 be so easily recog^iised in masses of fluids, as it can in solids. 



