A NEW THEOEY OF HEARING. 329 



by virtue of its own elasticity towards its position of rest, 

 to be immediately carried beyond that position by the 

 movement of the fluid, below it ; while it is itself forcing 

 the upper fluid forwards thus depressing the portion DE. 

 The small arrows in fig. 1 show the directions in which each 

 portion of the membrane is moving at the moment, viz : — 

 AB and DE are moving downwards the points A, B, D, 

 and E being momentarily at rest, A and E in equilibrium 

 B and D at maximum tension. BC is moving upwards 

 with increasing tension and diminishing velocity ; CD 

 upwards with diminishing tension and increasing velocity. 

 At C is the point of no tension and maximum velocity. 

 The large arrow shows the direction in which the wave 

 will travel forwards. 



Eig. 2 shows in a generalised and simplified way the 

 movements of the fluids. In the upper canal the fluid is 

 moving from the central wave-region outwards in both 

 directions, i.e., forwards from N to M and backwards 

 from N to 0. In the lower canal the movements are the 

 opposite of these, i.e., into the central region N 1 from 

 M 1 in front and from O 1 behind. 



The movements of the fluids in each region are first 

 arrested by the elasticity of the membrane and then 

 reversed : that is, the kinetic energy of the moving fluid 

 is transformed (so to speak) into potential energy of 

 stretched membrane, and then again into kinetic energy 

 of moving fluid to be similarly transformed in a portion 

 of the membrane further along the canal. 



It may be asked why the disturbance is transmitted 

 along instead of across the canals ; incompressible fluids 

 in rigid vessels can obviously however only move in such 

 directions as the yielding areas of the walls allow : if fluid 

 is forced forwards in the upper chamber a corresponding 

 and equal movement of the fluid in the lower canal must 



