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 



/ = /.+/* (1) 



f„ denoting that part of f which is doing statical work, i. e. 

 bearing the weight of the vapour in the column ; and f d de- 

 noting that part of jf 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 zj denote the pressure of the dry air, then 



jo = OT +/ s +/ d (2) 



If the air be in equilibrium, f d = 0, and p = zj + f s , this is the 

 value of jo used in formula (II.) : but if we suppose f s = 0, i. e. 

 the whole of the vapour at any point to be employed in mov- 

 ing the column, then p - -as + f, fa becoming equal to /, the 

 whole force of the vapour; but from (2) it is plain that zs +f s 

 is the pressure to be used in the barometric formula; and in 

 this supposed case zs+f s = zs=p-f. 



This corresponds to the case of incipient motion. Intro- 

 ducing it into (I.) we find 



H = ioooo/«*. (i +i |) log tzl r , (in.) 



and from (II.) we obtain 



H = lOOOoWl + Jx \ fti ^r % ^i- (IV.) 

 V 493/ p+p'-f-f' fe p -/' v ' 



