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



and, in theory, the time period of the system is only limited by the density of the 

 material and by its modulus of elasticity. 



In practice, however, the travel of the moving parts cannot be indefinitely 

 decreased, for the deflections must remain of such dimensions as to be accurately 



measurable. 



The following diagram illustrates the application of the principles we have just 

 established to the construction of a recording instrument (see fig. 2). 



Fig. 2. Diagram of recording manometer. 



A cylindrical groove is cut half through the walls of the enclosure. The upper part, 

 P, of the cylinder thus obtained represents the piston of our indicator, and the lower 

 portion, S, the spring. Under the pressure of the explosion the piston P will be 

 forced outwards to an amount corresponding with the elastic compression of the 

 material of which the spring is made. This motion is transmitted to the exterior by 

 the rod R. 



The lever L, supporting the mirror, rests on the fulcrum F at 3 ; it is kept 

 against the knife-edge 2 of R by the tension of the wire W. The wire W is of 

 considerable length, and is stretched almost to its limit of elasticity. The lever L can, 

 therefore, follow the small advance of the rod R without greatly diminishing the 

 tension of the wire W. The mirror focuses a point source of light on to a rapidly 

 revolving cylinder, thus recording on a magnified scale the motion of the piston P. 



It is not impossible that an indicator of this type would work in practice, but the 

 deflection of the mirror, and, therefore, the scale of the records obtained, would be 

 much too small. To increase the deflections, three modifications are necessary the 



