﻿Ignition of Gases by Sudden Compression. 87 



a uniform and exactly known motion across the plate 

 horizontally. To obtain this uniform horizontal velocity, the 

 arrangement in fig. 5 was adopted. 



A is a vertical cylinder closed at the bottom and carrying 

 at the top a guide for the piston rod B. At the lower end 

 of the rod B is a light loosely fitting piston in which are 

 drilled one or more small holes. The rod B has a collar C 

 above the guide, and can be loaded with weights, up to 

 50 lb. if need be. The cylinder is filled with paraffin 

 which, when the piston and weights have been raised and 

 then released by a trigger arrangement, is forced at high 

 velocity through the holes in the piston from the under to 

 the upper side. This arrangement gives a velocity of fall of 



the piston which is uniform to within 1 or 2 per cent., and 

 it was found, moreover, that this velocity varied directly as 

 the square root of the weight carried : a result which shows 

 that the viscosity of the oil is a negligible factor in deter- 

 mining the rate, and that the latter will therefore not be 

 affected by any small changes of temperature which might 

 occur from day to day in the oil. 



This point was checked experimentally, and it was found 

 that the maximum change in the rate of fall produced by 

 heating the oil from 19° C. to 65° 0. was 1|- per cent. As 

 the oil temperature during the experiments never varied by 

 more than 2° or 3°, the effects of temperature changes were 

 quite negligible. 



Piston and weights are suspended by a steel wire from an 

 arrangement of pulleys (F) carried on spindles attached to 



