64 
THE NATURE AND 
[sect. II. 
To determine the pressure at higher temperatures with the apparatus, the end K 
of the tube M N K was inserted at E; and as the temperature increased, the 
pressure of the steam in the cistern L caused the mercury to ascend, and con 
sequently afforded a means of measuring the amount of expansive force. 
The objection to this mode of trial is, that the temperature of the mercury must 
be continually changing during the trial, and steam must be either condensing or 
generating on its surface during the time of observation. At each observation the 
temperature of the whole of the apparatus ought to be the same, and then the 
column exhibiting the pressure ought to be reduced to its equivalent at the mean 
temperature. The only observation where these circumstances would have place 
was that which appears to have been made when the thermometer was at 42°; 
then the column in the syphon was 29*7, and the barometer stood at 29’84 : the 
difference is the force of steam at 42°, and is 014 inches. By cooling down to 32° 
the force v r as not perceptibly different, and we know r from later trials that this is 
nearly correct. Professor Robison, however, seems to have thought it w r as neces 
sary to have the force 0 at 32°. 1 
Robison’s Experiments on the Force of Steam. 
Temperature of 
the steam. 
Force of steam in inches of mercury. 
Temperature of 
the steam. 
Force of steam in inches of mercury. 
By Dr. Robison's 
experiments. 
By our Rule, 
page 59. 
By Dr. Robison’s 
experiments. 
By our Rule, 
page 59. 
32° 
0-0 
0-172 
160° 
8-65 
10-05 
40 
0-1 
0-245 
170 
11-05 
12-6 
50 
0-2 
0-37 
180 
14-05 
15-67 
60 
0-35 
0-55 
190 
17-85 
19-35 
70 
0-55 
0-78 
200 
22-62 
23-71 
80 
0-82 
1-106 
210 
28-68 
28-86 
90 
1*18 
1-53 
220 
35-8 
34-92 
100 
v 1-6 
2-08 
230 
44-5 
42-0 
110 
2-25 
2-79 
240 
54-9 
50-24 
120 
3-0 
368 
250 
66-8 
59-79 
130 
3-95 
4-81 
260 
80-3 
70-8 
140 
5-15 
6-21 
270 
94-1 
83-45 
150 
6-72 
7-94 
280 
105-9 
97-92 
If the elastic force *14, from which Robison began to register, had been added to 
all the experiments below 212°, as it ought to have been, they would have agreed 
extremely near with the results of later experiments. The experiments made by 
Achard seldom vary more than a degree or two from those in the above table. 
96. Mr. Dalton’s inquiries w r ere conducted by a different method. He took a 
1 Meehan. Phil. voi. ii. p. 36.