7 
516 E. P. TAYLOR. 
until at the instant of stopping it has increased to about 
ten times the value. This is evidently due to the lubri- 
cating film having broken down at these low speeds allow- 
ing an increasing degree of metallic contact. 
Revs PER SECOND 
Fic 4. TIME in SECONDS 
To find the negative acceleration or retardation at a 
particular instant, say, at a speed of two revolutions per 
second, a tangent, Fig. 4, was drawn to the velocity-time 
curve at this speed. The retardation was given by the 
value of the tangent expressed in the proper scale units; 
that is, the instantaneous change of velocity divided by 
the time during which the change took place. 
As a description of calculating the moment of inertia of 
the engines, one case worked out for the National gas engine 
is here given. Fig. 5 shows one of the combined braked- 
free runs. This velocity-time curve was drawn from a 
chart taken with the high speed of the chronograph in gear. 
Tie scales of the curve are very open and consequently the 
curve is represented by two very-nearly straight lines. 
