SOIL MOISTURE AND TO EVAPORATION. 37 



beginning. This must be interpreted to mean that both of these soils 

 were able to supply water from below as rapidly as it was lost at the 

 surface. During the experiment the amount of water lost by these 

 samples was 17.69 grams for the 30 per cent soil and 19.58 grams for 

 the 40 per cent soil, or 59 and 49 per cent of the original moisture con- 

 tent, respectively. 



A similar vessel, containing at the start 100 cc. of water, was 

 included in the same series with these soils. Its curve in general follows 

 very closely that of the 40 per cent soil and it is omitted from the figure 

 for the sake of simplicity. The fact above pointed out that the two 

 soils with greater moisture content can supply water as rapidly as it is 

 lost by evaporation is again clearly indicated by the observation that the 

 curve of loss from the water surface is practically coincident with that 

 of the 40 per cent Soil. The actual evaporating surface of the soil films 

 is probably larger than that of the water, but this difference is practi- 

 cally overcome by the slower diffusion of the water vapor as soon as the 

 evaporating surface penetrates at all below the surface of the soil. 



The average hourly rate of evaporation during this experiment, for 

 each square centimeter of general soil surface, was 0.0055 cc. for the 

 20 per cent soil and 0.0077 cc. for the 30 per cent soil. Of these two 

 soils the one with the greater moisture content was able to transmit 

 water at a rate at least as great as 0.0077 gram per hour for each square 

 centimeter, while the drier soil could not transmit water at a rate as great 

 as 0.0055 gram per hour, since the latter soil was unable to maintain its 

 average rate, but showed a rate which fell continuously. This point is 

 interesting in connection with the power of the soil to deliver water to 

 plant roots. 



