﻿86 Messrs. H. T. lizard and D. R. Pye on the 



standard Hopkinson optical type *, and need not be described 

 in detail. The piston used throughout the experiments was 

 one of 0*125 sq. in. area. Pressure on the piston deflects 

 a spring attached to it, and thus tilts a small mirror which 

 reflects a point of light from a fixed lamp. The magnitude 

 of the pressure is thus arranged to correspond to the vertical 

 (downward) deflexion of the image of the light on a 4£ x 3^ 

 photographic plate carried in a camera fixed to the indicator. 

 The calibration of the pressure scale was made in two 

 ways. Firstly, by subjecting the piston to known oil 

 pressures and measuring deflexions on the camera screen, 

 and, secondly, as a check on the piston area, by direct dead- 

 weight loading of the spring and measurement of the 

 deflexion. The oil pressures in the first calibration were 

 produced by a carefully gauged vertical plunger loaded with 

 weights. The calibration by this method was carried out at 

 the beginning of the experiments, and it was found that the 

 relation between pressure and deflexion was given very 

 closely by a straight line at a slope corresponding to a 

 pressure rise of 188 lb. per sq. in. per inch deflexion. At 

 the conclusion of the experiments this calibration was 

 checked by the dead-weight calibration as follows :; — 



Keading on Camera Screen. 



Load on Spring. Loading. Unloading. 



0-39 in. 040 in. 



Weight carrier = 325 lb 0-54 „ 0-54,, 



Carrier + 10 = 13-25 „ 0*975 „ 0"99 „ 



„ + 20 =23-25 ,', 1-40 „ 143 „ 



„ + 30 =33-25 „ 1-81 „ 1-81 „ 



This also shows a satisfactorily straight-line relationship 

 which checks remarkably well with the previous calibration, 

 for '^^'9 



Dead weight per inch deflexion = , ~ — 234 lb. 



corresponding to a pressure on a piston of area 0*125 sq. in." 

 of 8 x 23-4 = 187-2 lb. 



The previous figure of 188 lb. per sq. in. per inch de- 

 flexion has been used throughout the calculations. 



(2) The Timing Gear. 



The pressure in the cylinder being given by the vertical 

 motion of a point of light on a photographic plate, it was 

 necessary for the accurate measurement of the lengths ot 

 the delay period to give the point of light at the same time 



* Hopkinson, Proc. Inst. Mech. Eng. Oct. 1907, p. 863. 



