FRICTIONAL LOSSES IN INTERNAL COMBUSTION ENGINES. A499 
Measurement of the retardation may be made in a number 
of ways. The most suitable way for very accurate results 
is by means of a chronograph. 
For these tests there was mounted on the main shaft of 
the engine, an electric contact maker of a special design, 
originally devised by Professors Dalby and Callender. Hvery 
revolution contact was made for an instant, this completed 
an electric circuit round a small electro-magnet actuating 
a stylus on the chronograph. A sheet of glazed paper 
wrapped round the cylinder of the chronograph, and smoked, 
served to take the record of the revolutions, as shown by 
a series of kicks in the line scratched on it by the stylus. 
The cylinder is made to revolve by clockwork at either of 
two speeds; a low speed which moves the paper at a peri- 
pheral speed of 1 cm. per sec., and a high speed of 10 cm. 
per sec. At the same time the stylus is fed along the 
cylinder at a pitch of about8 mm. A retardation chart 
produced on this instrument shows a series of kicks at 
gradually and continuously-increasing intervals, as the 
_ speed of the engine falls. 
In the appendix will be seen how velocity-time curves 
were obtained from these charts, and how from the curves, 
by drawing tangents at various points, the retardations in 
revolutions per second per second were obtained at speeds 
of the engine corresponding to the points on the curve at 
which the tangents were drawn. It was estimated that 
these retardations were measured with an accuracy of half 
per cent. when using the high speed of the chronograph, 
and between two and three per cent. when using the low 
speed. 
3. Determination of Moments of Inertia.—To find the 
moment of inertia of the revolving masses was a serious 
difficulty in the early stages of the experiments, and a 
great deal of time was spent in developing a method of 
