358 Mr, Powelfs Appendix to M. Ramond*s Instructions [Nov, 



under the pressure F,were composed entirely of aqueous vapour, its 



weight would be j^ of the former ; that is to say, — ^—, Now 



we know by very decisive experiments that in a mixture of 

 vapour and air, which has attained a state of stable equilibrium, 

 these two fluids are uniformly diffused throughout the whole 

 space which they occupy. Tnus the weight of the mixture in 

 the preceding proportions will be equal to the sum of the weights 

 of tne air and vapour which occupy the given space under the 

 pressures A — F and F ; that is to say, that this weight will be 



h—'F 10 o F 7 



p . —T-- + ~ ~— , or simply p . — jj — . Now before the intro- 

 duction of the vapour, the weight of the same volume of dry air 

 submitted to the same pressure A, would be represented by p. 

 The densities being proportional to the weights, if 5 represent 

 the density of th6 stratum in the dry state, the density in the 



(a-?-f) gj, 



moist state will become S . , or 8 . 1 — -- - the 



pressure remaining the same. Thus we see that the introduc- 

 tion of aqueous vapour in the strata of air makes the ratio 



-, or C, vary proportionally to ( 1 — « ^ ) • 



The tension F is always very small at those temperatures at 

 which barometrical observations are commonly made. Its value 

 in metres for the point of extreme saturation may be calculated 

 from a formula, given by Laplace, from the experiments of 

 Dalton ; whence we find. 

 At 0° centigrade F = 0-005122 metre; (== 0*20165 inch.) 

 At 30° centigrade F = 0-031690 metre: (= 1-24765 inch.) 



and within these limits, which are nearly those of barometrical 

 observations, the increase of F may be sufficiently well repre- 

 sented by arithmetical progression, and will be 

 F=0-005122m. +0-0008649m. t (=0-20165 in. + 0-03304 Oin. 

 t being the temperature centigrade. Although this formula is 

 not rigidly accurate, it is sufficiently so in practice on account 

 of the little effect which it has on the observed heights. 



But before it can be applied to the state of the atmosphere, it 

 requires to be modified. It relates to the point of extreme satu- 

 ration at which the atmosphere is scarcely ever found ; and 

 consequently the value of F will almost always be rather greater 

 than the truth. No general determination can be given of the 

 quantity of vapour suspended in the atmosphere. This quantity 

 is extremely variable on different days ; it varies even from one 

 stratum to another in a manner very irregular, and often abrupt, 

 »9 we see on mountains where strata very little charged with 



