416 THE SENSORIAL FUNCTIONS. 



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,* suffers a momentary compression, 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 sonorous undulations 

 have an analogy to 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 sonorous undulations 

 spread out in all directions, forming, not circles 

 in one plane, but spherical shells ; and, what- 

 ever be the intensity of the sounds, the velocity 

 with which the undulations 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 

 miles in a minute : it is greater in dense, and 

 smaller in rarefied air ; being, in the same 

 medium, exactly proportional to the elasticity 

 of that medium. 



* The particles of water are as elastic, within a limited dis- 

 tance, as those of the most solid body ; although, in consequence 

 of their imperfect cohesion, or rather their perfect mobility in all 

 directions, this property cannot be so easily recognised in masses 

 of fluids, as in solids. 



