﻿90 



Messrs. H. T. Tizard and D, R. Pye on the 



which produced a horizontal movement of the point of light 

 more than twice as great as was necessary to traverse the 

 photographic plate, so that ample time was provided for the 

 speed to have become uniform before a pressure record was 

 taken. The fact that the indicator and all gear except the 

 actual falling weight was carried on the cylinder head, made 

 it possible to raise or lower this when changing the com- 

 pression ratio without affecting the timing gear. The only 

 difference was an alteration of about an inch in the distance 

 fallen by the weights between the position for maximum and 

 minimum compressions. 



Figs. 6 and 7 are prints taken from two typical records. 



Fiff. 6. 



I 



J 





OS 



7iW 



Secon3s 



o-ys- 



In the first there is no explosion, and the record shows simply 

 the rise of pressure due to compression, and subsequent fall 

 as the compressed mixture cools. Fig. 7 shows the first 

 rise of pressure due to compression, the " delay " period of 

 nearly constant pressure followed by the practically instan- 

 taneous rise on explosion, and finally the rapid cooling of 

 the intensely hot products of combustion. It may be 

 mentioned that the spring of the indicator was protected from 

 the force of the explosion pressure by a stop which prevented 

 the deflexion from ever being greater than just to the edge 

 of the photographic plate. 



J II. If a gas at a known temperature and pressure is 

 suddenly compressed in a gas-tight cylinder, we can calculate 

 from a measurement of the maximum pressure reached the 

 average temperature of the gas at the moment of maximum 



