Manchester Memoirs, Vol. xlvi (1901), No. 5. 7 



is enclosed in a light-tight box, a long narrow slit (about 

 -V' width) runs the entire length of the box parallel to 

 the axis of rotation. One of the filaments of an incan- 

 descent lamp is focused by the mirror on to this slit, 

 forming a fine straight line perpendicular to the axis of 

 rotation and to the slit. The sharp point of light thus 

 formed on the film moves from right to left as the pressure 

 increases. To secure the quality and intensity of light 

 which is necessary, the lamp is run at twice its normal 

 voltage at the moment of the explosion. 



To avoid the blurring of the zero line, the light is cut 

 off an instant later, and the zero marked in when the 

 products of the explosion have had ample time to cool to 

 atmospheric temperature. 



The gauge is calibrated by hydraulic pressure, and 

 the results are checked by comparison with the values of 

 the maximum pressure obtained by the statical method 

 described at last year's meeting of the British Associa- 

 tion*. 



For each explosive mixture two records are taken, one 

 at a high speed giving the rise of the pressure, the other 

 at a low speed giving the rate of cooling. 



Curves typical of these two records are giv'en in Figs. 

 3 and 4 {Plate io)-f-; they refer to a mixture of air and 

 coal gas, fired at an initial pressure of about eleven hundred 

 pounds per square inch. Oxygen was slightly in excess, 

 the residual gases containing about two per cent, of it. 



In tracing out diagrams like these, no part of the 

 instrument, except the light frame carrying the mirror, 

 moves more than one- or two-thousandths of an inch. 



* Report Brit. Assoc, 1900, p. 655. 



+ The original films can be measured to about one-thousandth of an inch ; 

 the prints given in figs. 3 and 4 have lost somewhat in sharpness, but are still 

 sufficiently good to give a fair idea of the records. 



