66 
MR. JOULE ON THE MECHANICAL EQUIVALENT OF HEAT. 
A large wooden screen (not represented in the figure) completely obviated the 
effects of radiant heat from the person of the experimenter. 
The method of experimenting was simply as follows : — The temperature of the fric- 
tional apparatus having been ascertained and the weights wound up with the assist- 
ance of the stand h, the roller was refixed to the axis. The precise height of the 
w'^eights above the ground having then been determined by means of the graduated 
slips of wood h, k, the roller was set at liberty and allowed to revolve until the 
w'eights reached the flagged floor of the laboratory, after accomplishing a fall of 
about 63 inches. The roller was then removed to the stand, the weights wound up 
again, and the friction renewed. After this had been repeated twenty times, the ex- 
periment was concluded with another observation of the temperature of the appa- 
ratus. The mean temperature of the laboratory w’as determined by observations 
made at the commencement, middle and termination of each experiment. 
Previously to, or immediately after each of the experiments, I made trial of the 
effect of radiation and conduction of heat to or from the atmosphere, in depressing 
or raising the temperature of the frictional apparatus. In these trials, the position of 
the apparatus, the quantity of water contained by it, the time occupied, the method 
of observing the thermometers, the position of the experimenter, in short everything, 
with the exception of the apparatus being at rest, w^as the same as in the experiments 
in which the effect of friction was observed. 
1st Series of Experiments . — Friction of Water. Weight of the leaden weights along 
with as much of the string in connexion with them as served to increase the pressure, 
203066 grs. and 203086 grs. Velocity of the weights in descending, 2'42 inches per 
second. Time occupied by each experiment, 35 minutes. Thermometer employed 
for ascertaining the temperature of the water, A. Thermometer for registering the 
temperature of the air, B. 
Table I. 
No. of experiment 
and cause of change 
of temperature. 
Total fall of 
weights in 
inches. 
Mean 
temperature 
of air. 
Difference be- 
tween mean of 
columns 5 and 6 
and column 3. 
Temperature of apparatus. 
Gain or loss of 
heat during 
experiment. 
Commencement 
of experiment. 
Termination of 
experiment. 
1 Friction 
1256-96 
5?698 
2-252 — 
55-118 
53-774 
O 
0*656 gain 
1 Radiation ... 
0 
57-868 
2-040 — 
55-774 
35-882 
0-108 gain 
2 Friction 
1235-16 
58-085 
1-875 — 
53-882 
56-539 
0-657 gain 
2 Radiation ... 
0 
58-370 
1-789- 
56-339 
36-624 
0-085 gain 
3 Friction 
1253-66 
60-788 
1-596- 
58-870 
59-515 
0-645 gain 
3 Radiation ... 
0 
60-926 
1-373 — 
59-515 
39-592 
0-077 gain 
4 Friction 
1252-74 
61-001 
1-110 — 
59-592 
60-191 
0-599 gain 
4 Radiation ... 
0 
60-890 
0-684 — 
60-191 
60-222 
0-031 gain 
1 
2 
3 
4 
3 
6 
7 
