MR. JOULE ON THE MECHANICAL EQUIVALENT OF HEAT. 
75 
The leaden weights amounted to 406099 grs., from which 2857 grs., subtracted for 
the friction of the pulleys, leaves 403242 grs. The mean height from which they 
fell, as given in Table II., was 1262*731 inches, from which 0*152 inch, subtracted for 
the velocity of fall, leaves 1262*579 inches. This height, combined with the above 
weight, is equivalent to 6061*01 foot-lbs., which, increased by 16*929 foot-lbs. on 
account of the elasticity of the string, gives 6077'939 foot-lbs. as the mean force em- 
ployed in the experiments. 
=773*762 ; which is therefore the equivalent derived from the above expe- 
riments on the friction of mercury. The next series of experiments were made with 
the same apparatus, using lighter weights. 
^rd Series of Experiments . — Friction of Mercury. Weight of the leaden weights 
and string, 68442 grs. and 68884 grs. Velocity of the weights in descending, 1*4 inch 
per second. Time occupied by each experiment, 35 minutes. Thermometer for 
ascertaining the temperature of the mercury, C. Thermometer for registering the 
temperature of the air, B. 
Table V. 
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 
1292*12 
49*539 
6*399 + 
49*507 
50*370 
0*863 
gain 
1 Radiation 
0 
50*165 
0*226 + 
50*370 
50*413 
0*043 
gain 
2 Friction 
1292*00 
49*865 
0*189 + 
49*606 
50*503 
0*897 
gain 
2 Radiation 
0 
50*363 
0*159 + 
50*503 
50*542 
0*039 
gain 
3 Friction 
1293*18 
50*139 
0*460 + 
50*168 
51*030 
0*862 
gain 
3 Radiation 
0 
50*617 
0*408 + 
51*030 
51*021 
0*009 
loss 
4 Radiation 
0 
50*750 
0*146 + 
50*873 
50*920 
0*047 
gain 
4 Friction 
1293*25 
51*401 
0*013 — 
50*920 
51*856 
0*936 
gain 
5 Radiation 
0 
49*936 
0*121 + 
50*031 
50*083 
0*052 
gain 
5 Friction 
1294*92 
50*551 
0*020- 
50*083 
50*980 
0*897 
gain 
6 Radiation 
0 
50*638 
0*135 + 
50*752 
50*795 
0*043 
gain 
6 Friction 
1294*43 
51*172 
0*065 + 
50*795 
51*680 
0*885 
gain 
7 Radiation 
"o 
51*553 
0*260- 
51*237 
51*349 
0*112 
gain 
7 Friction 
1294*07 
52*194 
0*371 — 
51*349 
52*298 
0*949 
gain 
8 Friction 
1293*30 
52*774 
0*019 — 
52*298 
53*212 
0*914 
gain 
8 Radiation 
0 
53*029 
0*204 + 
53*212 
53*255 
0*043 
gain 
9 Friction 
1294*05 
51*513 
0*306 + 
51*379 
52*259 
0*880 
gain 
9 Radiation 
0 
52*093 
0*177 + 
52*259 
52*281 
0*022 
gain 
10 Friction 
1293*95 
51*197 
0*180 + 
50*907 
51*847 
0*940 
gain 
10 Radiation 
0 
51*960 
0*079- 
51*847 
51*916 
0*069 
gain 
11 Friction 
1292*80 
50*577 
0*652 + 
50*804 
51*654 
0*850 
gain 
11 Radiation 
0 
51*055 
0*577 + 
51*654 
51*611 
0*043 
loss 
1 
2 
3 
4 
5 
6 
7 
L 2 
