46 Mr. Tredgold on the Theory of Evaporation. 
the temperature ¢. Then, if it be ascertained by experiment 
that the evaporation per minute, from a surface of one foot, is 
a when w= 1; we have 1:a::w:aw = the evaporation 
when the weight of vapour required for saturation, at the tem- 
perature ¢, is w. 
Again: Let e be the evaporation in grains that produces a 
aw 
depression of one degree of temperature, then T —¢ = ——; 
or T=ft+ <= . This is, however, not strictly accurate, un- 
less the specific heat of bodies be equal at all temperatures. 
The weight of a cubic foot of vapour at the temperature 60°, 
and pressure 30 inches, is 329°4 grains, and if f be any other 
990° is 
force, 30: f:: 329°4: aM = 10°98 f = the weight of a 
cubic foot of the force f and temperature 60°. And at the 
1-98 x510f _ 500F yearly. That is, the 
temperature ¢, 
450+¢t  — 450+¢ 
weight of a cubic foot of vapour at the pressure f and tempera- 
. 5600 , 
ture z 1s oye grains. 
The expansion of dry air by saturating it with moisture ap- 
pears to-be equal to the addition of the same volume of vapour, 
of the force it would have in a vacuum at the same tempera- 
ture, but both reduced to the same pressure. Therefore, if p 
be the greater pressure or force, and p! the less, the spaces 
being inversely as the forces 
‘ py 
P 
= the volume of the rarer fluid 
PSE A yh LCS 
corresponding to the greater pressure, consequentl rae) + 
p g g p ’ q Y. Bip 
U 
vs, (7) = the volume as increased by expansion. 
If the air be so rare that its force is less than that of steam 
of the same temperature, then p! indicates the force of the air ; 
but whenever the elastic force of the air exceeds the force of 
steam for the same temperature, then p = the force of the air. 
When the forces are the same, or p' = p, the volume is 
doubled by expansion. 
General Roy’s experiments, as far as they go, accord very 
well with this formula. The comparison of these experiments 
made by Mr. Daniell is not, however, quite correct. The 
volume of the air ought to be its volume at the same tempera~ 
ture as the vapour, and not increased after the operation for 
expansion, as he has done in his Essays, p.176. An example 
will render this more clear; and taking Mr. Daniell’s case 
(which 
