388 MR. J. E. PETAVEL ON THE PRESSURE OF EXPLOSIONS. 



It can, therefore, be obtained by differentiating the expression 



which was given on p. 385. 



Carrying out this operation we find 



The value of the ratio of the specific heats, y, is somewhat uncertain. For the 

 mixture of gases resulting from the explosion, y may be taken as 1'35 or 

 P21, according as the specific heats are considered constant or variable with 

 temperature. 



The following table gives the velocity of sound, calculated according to each of the 

 above hypotheses : 



VELOCITY of Sound in the Gases Produced by the Combustion of Cordite at the 

 Maximum Pressure of the Explosion, measured in Metres per Second. 



The limiting value for low densities, which should correspond with the speed of the 

 wave at the commencement of the explosion, works out at 1170 (y = 1'35) or 1108 



Although, strictly speaking, the above theory applies only to very small disturb- 

 ances, the calculated velocities are in fair agreement with the measurements given on 

 p. 387. 



The oscillations referred to in the preceding paragraph are superimposed on the 

 curve of pressure without directly altering its general shape. Within the limits of 

 the present experiments the wave action, consequent on the uneven distribution of 

 the charge, by increasing the thermal loss slightly lowers the maximum pressure. 

 The rate of combustion is, also, somewhat altered ; usually it is accelerated. 



These effects will be best understood by reference to figs. 14, 19, and 20, in which 

 the mean values of the pressure are plotted in terms of the time. 



