ELASTIC RADIAL DEFORMATIONS OF FLYWHEELS. 267 
fork is a small electromagnet which is included in the same 
circuit. This causes the fork to work automatically once 
it is set in vibration. Keys are inserted in each circuit as 
shown. Hach vibration of the fork corresponds to one 
hundredth of a second. Thus by counting the number of 
these corresponding to the interval between the points of 
making or of breaking of the wheel circuit, the time of one 
revolution may be found. For greater accuracy several 
such series may be counted and the average taken. By 
this means the speed at any instant can be obtained easily 
within one per cent. 
In making a test, the distance rod was first fixed in the 
desired position on the rim of the wheel. The points of the 
rod were set into two small chisel grooves, and a rubber 
band was slipped round this end of the rod in order to hold 
the points firmly in position. The holding down bands 
were then slipped round the rod and the cross-bar. The 
mirror was then moved about until the knife edge was 
visible in the telescope, and the zero reading taken. The 
driving belt was gradually pushed over, and the spindle 
and fork circuits closed, the fork being set in vibration. 
The observer then watched the movement of the knife 
edge on the micrometer scale, and, when he desired to 
take a reading of the deflection at any instant, he rotated 
the drum through about a third of a revolution, at the same 
time noting the reading on the scale. By this means the 
deflection and speed were observed at approximately the 
same instant. The curves traced on the smoked paper 
were fixed by dipping it into a trough containing a solu- 
tion of shellac in alcohol, and allowing it to dry. These 
cards were then worked up and the deflections plotted 
against speeds, for each particular position. From these 
curves, the true shape of the rim at any speed is obtained 
by plotting the deflection at that speed against angular 
