300 Journal of the Asiatic Society of Bengal. [N.S., XV, 
work on and increases the strain energy in the medium, by an 
amount which can be found as follows. 
Consider the medium with the reflecting surface at S (fig. 
1). Let the medium B to the left, be undisturbed, or in the 
case of a gas, at the normal pressure ; and let the medium A 
to the right be in vibration. During an interval of time, short 
compared with the period, we may suppose the strain in 
the medium A to remain constant, while the surface S moves 
to S’, a distance short compared with the length of the inci- 
dent waves. 
As an example we will treat with the case of sound waves 
inagas The pressure at any instant on a stationary reflector 
at Sis the difference between the pressure in A, and the normal 
pressure in B, 
Now let the reflector move to 8S’. The gas in B has re- 
mained undisturbed, and that now in SS’ is also at the normal 
pressure. To find the increase in the energy of the gas, i.e. 
e work done by the reflecting surface, it is convenient to 
Ss Se .S 
; | 
Fig. 1. 
consider this condition as produced in a somewhat different 
manner. 
Let SC represent the length occupied by the gas, which 
when brought to the normal pressure occupies SS’. Then let 
the reflector remain at S, while another screen, impervious to 
the gas, is inserted at C, and is then moved to S’. The gas in 
SS’ is now at the normal pressure, and the reflector can be 
removed, and substituted for the screen at S’. We have now 
the same conditions as those given by the advance of the re- 
flector from S to S’, and the work done on the gas will be the 
same in each case. 
We can investigate the work done with the aid of a dia- 
gram (fig. 2). 
e figure represents the conditions when A is rarefied. 
The heights of the horizontal lines above XY represent the 
pressures. e normal pressure is DN, the pressure in A is 
N. The instantaneous pressure on the reflector, due to the 
wave, is LM and is negative. 
