ee 
343 
and the air has returned to a state of equilibrium, then the 
observed barometric pressure is correct, and no deduction is to 
be made from it in consequence of the pressure of vapour. This 
is the supposition made in (II.), or the statical formula. 
In the present state of our knowledge, I fear it is impossi- 
ble to form a dynamical hygrometric correction for the baro- 
metric formula, but the principles on which it depends may be 
thus stated. 
Let f denote the observed elastic force of the vapour at 
any point; this quantity is the sum of two elastic forces 
Past ie (1) 
Ff; denoting that part of f which is doing statical work, i. e. 
bearing the weight of the vapour in the column; and f;, de- 
noting that part of f which is doing dynamical work, i. e. lift- 
ing and expanding the column of air. 
The barometric pressure at any point is therefore the sum 
of three quantities, viz., the pressure of the dry air, the sta- 
tical pressure of the vapour, and the dynamical pressure of 
the vapour. Let adenote the pressure of the dry air, then 
p=atfert fa (2) 
If the air be in equilibrium, f,; = 0, and p= a+ f,, this is the 
value of p used in formula (II.): but if we suppose f,=0, i.e. 
the whole of the vapour at any point to be employed in moy- 
ing the column, then p = a + f, fa becoming equal to f, the 
whole force of the vapour; but from (2) it is plain that a +f, 
is the pressure to be used in the barometric formula; and in 
this supposed case a + fs = a =p —f. 
This corresponds to the case of incipient motion. Intro- 
ducing it into (I.) we find 
H = 100002. (1 +385) ee — (IIL) 
and from (II.) we obtain 
H = 10000 / (1 + pry lop Zo, avi) 
