ON THE MECHANICAL EQUIVALENT OP HEAT, 
305 
found sufficient to carry off the heat, but, when larger powers were used, these 
sources of escape failed to keep the temperatures down to practical limits, which 
necessitated the application of currents of water to carry off the heat. 
The measurement of the work was invariably accomplished by attaching the brake 
blocks, or straps, to a lever, or arm, so tha,t the whole brake would be free to revolve 
with the brake-wheel, except for the moment of the weight of the parts which, 
adjusted to the power of the engine, was kept in balance by the adjustment of the 
pressure ot the blocks on the wheel. Then, since the work done is equal to the 
product of the mean moment of resistance, over the angle turned through, multiplied 
by the angle, if the resistance is constant over time, the moment of the brake, 
multiplied by the whole angle, measured the work done. 
It is however to be noticed that the assumption that the time-mean of the moment 
on the brake is the same as would be the angle-mean of this moment might involve 
an error of any extent, provided the resistance and the angular velocity varied in 
conjunction. And as steam engines invariably exert an effort within the period 
of the revolution while the friction and the pressure causing it are apt to respond to 
any variations of speed, it is probable that there has been some error from this cause 
in all such measurements although not previously noticed. 
Hirn ajDpears to have been the first to recognise that in a steady condition the 
resistance of fluid between the brake-wheel and the brake would answer instead of 
the solid friction, so that the mean time moment of effort exerted in turning a 
paddle in a case with bafflers containing water would be strictly measured by the 
mean time moment of the case. And although subject to the same error from periodic 
motion as the friction brake, the facility this fluid brake offered for cooling and 
regulating led to its simultaneous adoption and development by several inventors, 
for measuring power—the late William Froude, for the purpose of measuring the 
work of large engines, inventing that arrangement of paddle vanes and bafflers 
which gives the highest resistance, regulating the resistance by thin sluices between 
the vanes and bafflers, and always working with the case full of water. 
The brake under consideration differs from that of Mr. Froude in only one funda¬ 
mental particular—the provision by which a constant pressure in the interior of the 
brake is secured by the admission of the atmosphere to that part of the brake where 
the dynamical effect of the water is to cause the lowest pressure—this admits of 
working the brakes with any quantity of water from nothing to full, and thus allows 
of the regulation of the resistance by regulating the quantity of water in the brakes 
without sluices. 
The description of this brake has already been published, together with that of the 
engines,* but it will be convenient to give a short description. 
This brake consists primarily of (1) a brake wheel, 18 inches in diameter, fixed on 
* “ Triple ExpansioD Engines,” by Professor Osboene Reynolds, ‘ Minutes of Proceedings, Inst. 
C.E.,’ vol. 99, 1889, p. 18. 
VOL, CXC.—A. 2 R 
