ELASTIC RADIAL DEFORMATIONS OF FLYWHEELS. 975 
the spindle. This jack-shaft was driven by belting from 
a series of three larger shafts, the last of which was 
driven by a leather belt from a6 HP. gas engine. Thus 
the effects of the variations of the speed of the gas engine 
were greatly minimised, and the running was found to be 
very uniform. By gearing up from each shaft the speed 
obtainable was about 1,500 revolutions per minute. For 
the purpose of this investigation, about 1,000 revolutions 
per minute were considered sufficient. The belt driving 
the jack-shaft was allowed to have a considerable amount 
of slip on its pulley, the surface of this pulley being occas- 
ionally sprinkled with kerosene. By this method the speed 
was raised very gradually, the belt being moved very slowly 
from its loose pulley on the jack shaft to the driving pulley 
by means ofalever. The time taken to increase the speed 
from zero to 1,000 revolutions per minute was from four to 
five minutes, thus allowing ample time fora series of obser- 
vations of speed and defiection to be taken. 
The speed was measured by means of an electric chrono- 
graph which recorded on a sheet of smoked paper the 
revolutions of the wheel together with the number of 
vibrations made by a tuning fork of known frequency. Thus 
since one vibration of the tuning fork represents a definite 
interval of time, the time occupied by one revolution of the 
wheel can be ascertained by counting the number of vibra- 
tions of the fork corresponding to one revolution of the 
wheel. The arrangement of apparatus is shown in the 
diagram (fig. 8). On the spindle of the wheel is clamped 
a one bar commutator M, against which pressed a steel 
spring acting as a brush. This commutator, therefore, 
made and broke the current in its circuit once for each 
revolution of the wheel. In this circuit is a small electro- 
magnet with a soft iron armature borne on a spring (S.). 
The action of this spring is to hold the armature apart 
