ON THE MECHANICAL EQUIVALENT OF HEAT. 
367 
The duties of the second assistant Mr. J. W. Matthews consisted in regulating 
the engine speed by means of the hand brake, more particularly at the commencement 
and end of each trial, and also in keeping a constant pressure of 9 inches of mercury 
in the artificial atmosphere. 
The boy’s time was occupied in breaking up the ice and feeding it as required into 
the ice cooler. 
In the last series of experiments three similar trials of 62 minutes duration each 
were made per day, and the engine having been once started was not stopped till the 
three trials were completed. Consequently what I say below as to the starting of 
the engine does not refer to every trial, for after emptying the tank at the close of 
any one all the necessaiy adjustments were ready made for the next. 
I. The pump and engine were started simultaneously, the brake being therefore 
supplied with a stream of cold water through the ice cooler. The brake then 
automatically adjusted the weight of contained water till the load floated clear of the 
engine floor. The speed was then adjusted till the speed indicator gave the required 
reading, viz., in all recorded trials 300 revolutions per minute. 
II. Since all the work done was expended on the stream of water passing through 
the brake, its final temperature rose more or less quickly, and by adjusting the 
regulating valve on the supply pipe the temperature of discharge finally remained 
steady at 212° Fahr. nearly. In the meantime the supply of water to the stuffing- 
box was regulated till the temperature of the cover was at the required level. 
These adjustments took from a quarter to half an hour, and when made, the 
engine was allowed to run for some half-hour longer to ensure a steady condition 
being attained. 
The water supply to the condenser had also been regulated till the stream of water 
issuing from the rising pipe and flowing to waste had the requisite temperature. 
III. Headings were then taken of— 
(a) The revolution counter. 
{h) The weight of the empty tank and its cover. 
IV. When a steady condition was reached, the revolution counter at a given signal 
was pushed into gear with the small spindle previously mentioned, making connection 
through the cap with the engine shaft, and simultaneously the two-way tipping 
switch, which had hitherto been directing all the water to waste, was pulled over and 
diverted the whole stream into the tank. In the later trials all leakage that did 
sometimes take place from the stuffing-box, and a slight leakage that always 
occurred at the automatic cock below the brake, were collected in two bottles kept 
for that purpose. These were put under the drain pipes in each case as soon as 
possible after the signal. 
The speed of the engine as indicated by the gauge was read when the signal was 
given, and as soon as possible afterwards a reading was taken of the temperature in 
the discharge pipe. 
