ON THE MECHANICAL EQUIVALENT OF HEAT. 
359 
cooler and the regulating valve controlling the main supply to the brake. It was 
consequently under considerable pressure and at a temperature very little over 
32° Fahr. The water thus supplied had, of course, to enter the brake, and the 
amount supplied afforded a very convenient means of controlling the temperatures of 
the bearings. 
At a distance of 2f inches from the cap of the stuffing-box was the end of one 
of the main bearings, Pt, carried on the cast-iron pedestal, S. 
It was important that I should have some control over the loss of heat by con¬ 
duction along this length of shaft. Accordingly, two pieces of brass pipe were 
soldered on to the cap of the stuffing-box, while two others were screwed, the one in 
the upper and the other into the lower brass forming the main bearing. Thermo¬ 
meters were placed inside the tube affixed to the stuffing-box cap, which happened 
to be uppermost at the time, and into the two pipes screwed into the main bearing. 
It was then assumed that the loss of heat along the shaft would vary with the 
difference of temperature between the stuffing-box cap and the bearing. In order 
that the losses of heat occurring in this way in any two trials should be identical, 
it was sufficient under the above assumption that this difference of temperature 
should be the same in both trials, and the temperature of the stuffing-box was 
regulated to this end by means of the amount of cold water passing into it. 
Considerable difference of temperature was observed between the upper and lower 
brasses of the bearing, and as it seemed probable that the lower one approximated 
the more closely to the temperature of the shaft, that thermometer was the one 
used in determining the loss of heat by conduction. 
In the later trials I endeavoured to keep the temperatures of the stuffing-box and 
the bearing at tbe same level, thus entirely eliminating this cause of loss from the 
experiments. 
Water Jackets for the Low and High L'emperatvre Thermometers .—(Part I., par. 15.) 
8 , It was evident that the temperatures of the water would be much more easily 
and accurately taken if the whole stem of each thermometer was kept at one 
temperature. To this end each of the principal thermometers was completely 
jacketted with a stream of the water whose temperature was required. 
The arrangements adopted for this purpose are illustrated in the annexed 
sketches. (Pigs. 3 and 4.) 
After leaving the main regulating valve the cold supply water entered a vertical 
brass T, shown at A. The main volume of the water flowed on to the brake 
through the horizontal arm of this T. At its upper end the T carried a small 
stuffing-box, B, into which was fixed a vertical .g-inch diameter glass tube, C. This 
tube was closed at its upper end by means of a rubber stopper, held in place by the 
brass cap, D, screwed on to the upper end of a |-inch slotted copper pipe surrounding 
