OBSERVATIONS ON DETERMINATIONS OF DROUGH I INTENSITY. 79 



5 — from the surface. Part of the heat of the surface layer of soil 

 is also abstracted by the heating of the superficial layer of air 

 above it, and by the distributive effect of the resulting convection 

 currents. It is easy to see, from the foregoing, what a significant 

 part is played by solar radiation in exhausting the moisture of soil: 

 it is that form of energy which contributes most directly and 

 to the conversion of the moisture with aqueous vapour. Water 

 vapour has a density as compared with dry air of only about 0*623 

 under the same conditions of temperature and pressure, so that 

 even with saturated air at ordinary temperatures, this difference 

 of density would promote considerable convection al motion, if not 

 counteracted by commensurate resistances arising from the great 

 interstitial surface-area of the soil. These resistances, however, 

 are so great as to make the effect of mere difference of density 

 probably negligible. 



1 5. Diffusion of aqueous vapour into the atmosphere. — Relatively 

 to the diffusion into the atmosphere of the aqueous vapour produced 

 from the moisture in the soil, the difference-of-density element 

 just considered is certainly negligible. As Graham's experiments 

 clearly shew, porous substances, even when sufficiently dense to 

 maintain as diaphragms very considerable differences of pressure 

 between two gases, scarcely affect the rate of interdiffusion. The 

 interstitial-surface resistance here therefore sensibly disappears, 

 and obviously the drying of soil must be regarded as mainly 

 dependent on the phenomenon of molecular diffusion. And since 

 its rate, other things being equal, varies probably nearly as the 

 square of the absolute temperature, the solar-radiation effect is 

 important as regards the soil, not only in respect of the conversion 

 of its water into aqueous vapour, but also, by increasing the 

 temperature, in promoting diffusion as the aqueous vapour is 

 produced. 



16. Effect of air temperature. — The influence of the temperature 

 of the air is at least twofold : on the one hand it affects the rate 

 at which the heat in the soil is dissipated : and on the other it 

 affects the rate of diffusion by altering the saturation value of the 



