74 
MR. J. P. JOULE ON THE AIR-ENGINE. 
It will be observed that the excess of the temperature of the room above the mean 
temperature of the water in the lower vessel, was, in the experiments with heated air, 
2 °' 42 , but in the experiments on the effect of radiation 2°‘459. A comparison of the 
several experiments with one another, furnished the means of determining the amount 
of the small correction due to this circumstance. Hence 0°'9254-0°‘002 — 0°'448 
=0°'479 will be the corrected mean increase of temperature due to the current of 
heated air. The material in which this increase took place consisted of 175500 grs. 
of water, 15635 grs. of copper, and 53370 grs. of lead, the whole having a capacity 
for heat equivalent to that of 178535 grs. of water. The volume of air passed through 
the pump was 12*77 X 26 X24 = 7968’48 cubic inches, which, at the observed baro- 
metric pressure and the temperature 50°*6, would weigh 2537'94 grs. We have 
therefore for the specific heat of atmospheric air at constant pressure — 
178535 X 0*479 
2537*94 X 146*45 
0*23008. 
Series II. — Pump worked 26', at the rate of forty strokes per minute. 
No. of 
Experi- 
ment. 
Source of calorific effect. 
Height 
of Ba- 
rometer. 
Tempe- 
rature of 
Baro- 
meter. 
Tempe- 
rature of 
Air- 
pump. 
Tempera- 
ture of 
upper 
vessel. 
Tempe- 
rature 
of the 
room. 
Temperature of the lower 
vessel. 
Increase 
of tem- 
perature. 
Commence- 
ment of Ex- 
periment. 
Termination 
of Experi- 
ment. 
1 
1 
2 
2 
3 
3 
4 
4 
5 
Radiation 
0 
0 
4^223 
47-558 
47 - 8 41 
48- 099 
48- 339 
49*107 
49- 524 
49- 850 
50- 030 
44-200 
44- 648 
45 - 902 
46- 319 
47- 516 
49 - 327 
50- 443 
50- 728 
51- 809 
41-648 
45 - 902 
46- 319 
47 - 516 
47-860 
50-443 
50- 728 
51- 809 
52- 037 
0- 448 
1- 254 
0-417 
1*197 
0- 344 
1 - 116 
0- 285 
1 - 081 
0-228 
Heated air and radiation ... 
Radiation 
30*6 
47-75 
52 
197-71 
Heated air and radiation ... 
Radiation 
30-602 
48-25 
53-5 
198-63 
Heated air and radiation ... 
Radiation 
30-61 
49-5 
55-4 
202-42 
Heated air and radiation ... 
Radiation 
30-607 
50-25 
56-4 
203-13 
Mean. 
Mean. 
Heated air and radiation ... 
Radiation 
30-605 
48-94 
54-32 
200-472 
48-653 
48-591 
47-755 
47-974 
48-917 
48-318 
1-162 
0-344 
In the above series l°*162-f 0°*006 — 0°*344 = 0°*824 will be the corrected mean 
increase of temperature due to the current of heated air. The material in which 
this increase took place consisted of 175000 grs. of water, 15635 grs. of copper, 
and 53370 grs. of lead, the whole having a capacity for heat equivalent to that of 
178035 grs. of water. The volume of air passed through the pump was 12*77X26 
X 40= 13280*8 cubic inches, which, at the observed barometric pressure and the 
temperature 54°*32, would weigh 4252*7 gi’s. Hence we have for the specific heat — 
178035 X 0*824 
4252-7 X 152*136 
=0*22674. 
By another series of experiments, in which the air-pump was worked at the velocity 
of twenty strokes per minute for twenty minutes, I obtained the value 0*2325. The 
mean of the three results is 0*22977, or nearly 0*23, which we may take as the specific 
heat of air at constant pressure determined by the above experiments. 
