CHAPTER III. 



THE EXTERNAL ENVIRONMENT (continued). 



173. The Influence of the Hygrometric State. The hygrometric 

 state or humidity of the air at a given temperature is the ratio of 

 the weight of water vapour which it contains to the maximum 

 weight which it could contain in the same volume. 



Let p and P be those two weights, the hygrometric state will be : 



4 



It is, in fact, the relative humidity. In different publications 

 the state of relative dryness or the relation =- is given. 



It is obvious that the state of dryness increases as that of p is 

 reduced. 



In practice the weights p and P of vapour are not considered, 

 but its tensions /and F, that is to say, its " elastic force," as it is 

 observed, and as it would be if it were the maximum, for a given 

 temperature t. The hygrometric state will be : 



-i 



the value of F can be found for all temperatures in Regnault's 

 tables (see Technique, 253) ; /is measured by means of a hygro- 

 meter, of which that of Crova is the simplest and best ; its prin- 

 ciple being the following : In cooling the vapour of the air, there 

 will be a temperature l' t at which the elastic force is a maximum ; 

 t' therefore will be found at the " dew-point," when the vapour 

 starts to condense. The tables give the force corresponding to 

 /'as/. 



Example : In a locality at 15 C. the dew point 01 the hygro- 

 meter is found when t' = 5 C. The tables give, for this tem- 

 perature : /== 6-55 mm. ; for i = 15, F = 12-70 mm. Hence : 



*=t =j|f = 0-514, or 514%. 



Knowing /or e we can calculate the weight of water vapour 

 per cubic metre at tC. 



290-2 X f 290-2 x F X e 



or = 



273+* ' 273 



