THE MECHANICAL EQUIVALENT OF HEAT. 
453 
Hence 
a- = *000355 tr^ = 7606 X 10^* 
p = *000399 
Q = *0138 p = *48 millims. 
The reduction of our observations involved so much arithmetic that the whole 
series of experiments was concluded before we were able to get the results in the 
form now given. 
Although we were prepared to find a rapid reduction in the value of Q as the 
pressure diminished, we had not realized the importance of small changes in the 
pressure. This was unfortunate, for, as the figures show, our average pressure v/as 
just that at which small alterations produced the greatest effect. We usually con¬ 
sidered it sufiicient to record the pressure each day, whereas it appears that a careful 
observation of the pressure during each stirring experiment ought to have been made. 
It is evident that we should have doue better to select pressures of not less than 
1 millim,, and thus small variations would have had but little effect. 
At the time of the observations we did not know the real value of the water 
equivalent. It was thus impossible for us to obtain Q with any accuracy, and it is 
only by the changes in the value of Q that the effects of changes in pressure are 
rendered evident. 
The results compare very favourably with the conclusions arrived at by Bottomley.* 
Collecting the numbers given in this section (with the exception of Experiments 72 
and 73, see note, p. 450), we get 
Table XXVI. 
Date. 
Experiment. 
Mass of water. 
Pressure in 
millims. 
Thermal grms. 
per second. 
Sept. 10-13 . 
83- 93 
139-78 
1-15 
•0140 
Aug. 8-10 . 
41- 50 
103-01 
1-15 
•0140 
„ 11 ... . 
51- 54 
103-01 
-98 
•0140 
Sept. 14-16 . . . 
94-102 
199-67 
-48 
•0138 
„ 16-18 . . . 
103-110 
259-50 
•48 
•0138 
Aug. 26 ... . 
79- 80 
277-93 
-44 
•0136 
„ 14-26 . . . 
58- 59 
188-07 
-40 
•0134 
„ 24, 25 . . . 
62- 65 
188-07 
-37 
•0132 
„ 30,31 . . . 
74- 78 
277-93 
-37 
•0132 
„ 26,27 . . . 
66- 71 
277-93 
-37 
•0131 
„ 17,18 . . . 
60- 61 
188-07 
-37 
•0131 
Sept. 4 . . . . 
81- 82 
140-27 
•36 
•0130 
These numbers are plotted in fig. 8 the abscissse denoting pressures and the ordinates 
the number of the thermal grms. gained per second. When it is remembered that 
adjacent points are obtained from very different masses of water, the close agi’eement 
is a proof of the accuracy of the observations. The sudden change in the value of Q 
* ‘ Phil. Trans.,’ 1887, A. 
