500 -E. P, TAYLOR. 
making this determination with the degree of accuracy 
required. . 
Many methods of finding the moment of inertia of a body 
are available, but when the body is heavy and not readily 
removed from its bearings the number is practically nar- 
rowed down to the following. 
Suppose a retardation test is run and from the chrono- 
graph there is obtained a certain value 1 for the retardation 
at a certain instant when the engine was revolving at N 
revolutions per second. 
At this instant Ty = Q2QrIny 
Now if a second test be made, but this time with a 
known brake torque Ty» applied to the engine shaft, the 
retardation at the instant when the speed is again N revolu- 
tions per second is greater than m, giving 
Ty + Tr = 27 Ing 
Assuming JT; to have remained unchanged, since both 
values are at the same speed, a combination of the two- 
equations gives 
Ty, = 271 (m2 - 1) 
Ty 
Se ee 
a 27 (m2 — 1) 
All the values of the right-hand side of the equation are 
experimentally known; hence, the value of “‘J,’’ the 
moment of inertia of the revolving masses, can be deter- 
mined. 
4. Type of Brake employed.—Too much stress cannot. 
be laid on the necessity for a very carefully-designed brake. 
by which this known brake torque is to be given to the 
engine. The tangential pull of the brake, and also the. 
lever arm at which it acts on the shaft must each be capable. 
of precise measurement. On this account brakes of the 
rope or band variety, which were at first tried, were- 
