80 G. H. KNIBBS. 



air itself ; since the quantity of aqueous vapour in saturated air 

 is a function of the temperature. Regnault's beautiful researches 

 furnish the necessary data for the determination of this total 

 amount. 



17. Humidity and its influence. — The rate of drying of a soil — ■ 

 depending as it has been said on diffusion — requires as the factors 

 for its determination not only the soil temperature, but also both 

 the air temperature, and either the relative or absolute humidity 

 of the air ; because of their reaction on the velocity of diffusion. 



18. Effect of Wind. — The effect of atmospheric motion on the 

 rate of drying is very striking, and is due to the acceleration of 

 diffusion by the rapid removal of the layer of air highly charged 

 with aqueous vapour, next to the soil ; so that for the deductive 

 determination of this rate, it is very necessary to take into account 

 the velocity of wind on the surface. Mr. Deane, in his diagram, 

 has made use of curves of evaporation, whose parameters were 

 empirically deduced from the rates of evaporation of water. The 

 tentative character of such a proceeding is, of course, fully admitted. 

 It is not at all improbable that direct and convenient relations, as 

 has been assumed, may be ascertained between these rates, and 

 those of the drying of definite classes of soil under definite condi- 

 tions ; so that in place of a complicated function to determine 

 drying, depending on radiation and on atmospheric temperature, 

 humidity and motion, it may be possible to substitute an empirical, 

 but sufficiently exact and relatively very simple one, depending 

 merely upon the rate of evaporation of water, subjected of course 

 to similar circumstances. This would greatly simplify the problem, 

 and the investigation would be of value. 



19. General expression for the degree of saturation. — The exhaus- 

 tion of the moisture of a soil is dependent, we thus see, on drainage 

 losses (p), and is a function of solar radiation (a), air temperature 

 (t), humidity (/jl), the coefficient of diffusion between air and aqueous 

 vapour (8), and the velocity of wind (w say). That is to say, the 

 saturation of soil, neglecting the effect of vegetation, is represented 

 in its entirety not merely by an expression like (4) but requires 



