226 Mr Anderson’s Corrections for the Effects of Humidity 
To determine the coefficients A and B, two equations of condition 
at least are necessary. The comparison of the results of a consi- 
derable number of observations, made under every variety of 
temperature and humidity, gave A = 36 and B — -- J_ : we 
A 
thus obtain, 
/= F- 
30 
( ye -w) 
or/- F 
| 6 S 
180 — r 
1 his expression, remarkable for its simplicity, will be found 
to give the elasticity of atmospheric vapour nearly the same as 
that formerly deduced. If we employ the same data as before, 
the barometrical pressure being 30.4 at the time of the observa- 
tions, we have 
f— -524 — l*.. 804 ' * &5 = .524 _ .240— .284. In. 
8.5 
180 “To 
The former result was .285 in., differing from the latter only 
.001 in. 
The elasticity of the atmospheric vapour being known, it is 
easy to derive from it the weight of the absolute quantity of 
moisture contained in a given volume of air. Let q> be the elas- 
tic force of the atmospheric vapour, cooled down from the actual 
temperature £, to a temperature r, at which it just begins to pass 
to the liquid state, then the vapour will be in the maximum 
state of tension for the temperature r. But, according to the 
experiments of Gay Lussac, vapours, so long as they retain their 
elastic condition, are expanded by heat, precisely in the same 
manner as the gases ; that is, .002086 of their volume, regard- 
ed as unity, for each degree of Fahr. 
Hence, vapour, whose maximum elasticity at the temperature 
r is <p, when raised to the temperature £, will have its elasticity 
increased to (p (l -f -002086. t — t) ; therefore, 
/= <P (1+-002086 t — r), and <f> = j + -00208b (* — *). 
Here, indeed, r is unknown ; but an approximation to it may 
