unted to only 5%, with the brake uncovered, and was reduced to less
than 2% by lagging. This is the special advantage of working on so
large a scale with so rapid a generation of heat. But, for the same
reason, the method necessarily presents peculiar difficulties, which
were not overcome without great pains and ingenuity. The principal
troubles arose from damp in the lagging which necessitated the
rejection of several trials, and from dissolved air in the water,
causing loss of heat by the formation of steam. Next to the radiation
loss, the most uncertain correction was that for conduction of heat
along the 4-in. shaft. These losses were as far as possible eliminated
by combining the trials in pairs, with different loads on the brake,
assuming that the heat-loss would be the same in the heavy and light
trials, provided that the external temperature and the gradient in the
shaft, as estimated from the temperature of the bearings, were the
same. The values deduced in this manner for the equivalent agreed as
closely as could be expected considering the impossibility of
regulating the external condition of temperature and moisture with any
certainty in an engine-room. The extreme variation of results in any
one series was only from 776.63 to 779.46 ft.-pounds, or less than
1/2%. This variation may have been due to the state of the lagging,
which Moorby distrusted in spite of the great reduction of the
heat-loss, or it may have been partly due to the difficulty of
regulating the speed of the engine and the water-supply to the brake
in such a manner as to maintain a constant temperature in the outflow,
and avoid variations in the heat capacity of the brake. Since hand
regulation is necessarily discontinuous, the speed and the temperature
were constantly varying, so that it was useless to take readings
nearer than the tenth of a degree. The largest variation recorded in
the two trials of which full details are given, was 4-9 deg. F. in two
minutes in the outflow temperature, and four or five revolutions per
minute on the speed. These variations, so far as they were of a purely
accidental nature, would be approximately eliminated on the mean of a
large number of trials, so that the accuracy of the final result would
be of a higher order than might be inferred from a comparison of
separate pairs of trials. Great pains were taken to discuss and
eliminate all the