316 
PROFESSOR 0. REYNOLDS AND MR. W. H. MOORBY 
brake, as well as its quantity, to obtain an approximate estimate of the heat equivalent 
of the work done. But the recognition of the extreme difficulty of obtaining any 
first-hand assurance as to the accuracy of scales of thermometers, and the fear of 
creating erroneous impressions as to the value of the equivalent, made me reluctant to 
allow such determinations. For this reason, as well as to avoid complicating the 
brake, in the first instance I made no provision for the introduction of thermometers, 
as may be seen in Plate 3. 
But, after the engines and brake had been in use for two years, and had been 
iound to possess attributes in steadiness of running, delicacy of adjustment and 
balance, beyond what I had dared to expect, and particularly in being able to work 
with an almost absolutely steady supply of water between steady temperatures, and 
the same temperatures for different powers, arising either from differences of speed, 
or differences of load, I realized that by working with the same thermometers on the 
same parts of their scales, and with the same loads and temperatures at different 
speeds, since the relative error of balance would be the same, if the surroundinc>’ 
tenq^eratures were the same, the difference of two trials would afford the means of 
determining the loss of heat by radiation, and, this being determined, the difference 
of two trials made at the same temperatures as the previous trials, and both at the 
same speeds, but with different loads, would afford data for determining the error of 
balance without introducing the value of the equivalent or the use of the scales of 
the thermometers, except to identify equal temperatures. 
I then yielded to the very general wish on the part of those who worked in the 
laboratory, and added such jarovision to the brake on the low-pressure engine as 
would admit of the measurement of the heat carried away by the effluent water, but 
only for the purpose of verifying the accuracy of balance as determined by mechanical 
means. 
The Thermal Verijicatioii of the Balance of the Brakes. 
10. The desirability of such independent determination of the balance arose in 
the first instance from the circumstances already described (Art. 9), viz., that the 
statical balance could only be determined to 1 ft.-lb., while the absence of eflect 
from the friction of the automatic gear, &c., was only arrived at by somewhat 
complicated considerations. 
The supply of water to the brake came from the service tank, 10 feet above the 
floor, and 7 feet above the shaft, the tank being supplied direct from the town main, 
and regulated by a ball-cock. The pipe from the tank passes beneath the concrete 
floor to a point conveniently close to the brake, whence a branch, in which is a hand- 
cock, rises vertically to a height of 4 feet above the floor, at which height is the 
automatic inlet valve, and from this the pipe is bent ov'er, so that its mouth is 
directly over the inlet opening into the brake, with which the pipe is connected by a 
flexible indiarubber tube. 
