and on Agronomical Refractions, 
513 
Pressure. 
Temperature. 
Error of 
calculated 
temp. 
Pressure. 
Temperature. 
Error of 
calculated 
temp. 
Observed. 
Calculated. 
Observed. 
Calculated. 
Inch. 
0 
O 
Inch. 
•52 
62 
59-5 
- 2*5 
4-68 
132 
131-4 
--6 
•73 
72 
69-3 
- 2-7 
6-06 
142 
141-3 
--7 
102 
82 
79-3 
- 2-7 
7-85 
152 
151-6 
--4 
1-42 
92 
89-9 
- 2-1 
9-99 
162 
161-7 
--3 
1-95 
102 
1002 
- 1-8 
12-64 
172 
171-8 
-•2 
2-65 
112 
110-7 
- 1-3 
15-91 
182 
1820 
0 
3-57 
122 
121-3 
- *7 
29-80 
212 
212 - 
The formula deviates slightly from the observations at very low 
pressures, Dalton says that it is next to impossible to free any li- 
quid entirely from air; of course if any air enter, it unites its force 
to that of the vapour. — Manchester Memoirs^ vol. v. p. 570. It 
must be recollected that according to theory the constants y and 
E are the same only as long as the chemical constitution of the 
vapour remains the same, and they vary for different substances. 
With regard to the nature of the accurate expression which con- 
nects the pressure with the temperature, opinions have hitherto been 
various. According to Dr. Robison, Mr. Watt found that w^ater 
would distil in vacuo when of the temperature of 70°, and that in 
this case the latent heat of the steam appeared to be aljout 100°; 
and some other experiments made him suppose that jthe sum of the 
sensible and latent heats is a constant quantity. This, Dr. Robi~ 
son says, is a curious and not improbable circumstance. Southern, 
on the contrary, concluded from experiments on the latent heat of 
steam at high temperatures that the latent heat is a constant quantity, 
instead of the latent heat + sensible heat being so. M. de Pambour, 
in speaking of Southern’s view, says, “ Cette opinion a paru plus 
rationelle a quelques auteurs, mais le premiere nous semble mise 
hors de doute par les observations que nous aliens rapporter.” It 
appears to me by no means clear that Watt entertained the opinion 
here attributed to him, for in a note in the Appendix to Sir David 
Brewster’s edition of Robison’s Mechanical Philosophy, vol. ii. p. 
167, he professes to agree in the opinion there delivered by Southern. 
In p. 166 Southern records three experiments, from which he ob- 
tained 1171°, 1212°, and 124<5°, for the sums of the latent + sensible 
heat corresponding to the temperatures or sensible heat 229°, 270°, 
295°. If we take the two extreme observations, we find a difference 
in the sum of the latent + sensible heat of 74 degrees, corresponding 
to a difference in the sensible heat of 66 degrees. 
If the conditions under which Laplace obtained the equation 
r = ^ 
e 
Phil. Mag. S. 3. Vol. 16. No. 105. June 1840. 2 M 
