SOIL MOISTURE AND TO EVAPORATION. 35 



movement through the soil films, thus producing a protecting air-dry 

 layer at the surface. The maximum rate of movement of liquid water 

 through a soil layer depends, first, upon the dimensions of the capillary 

 spaces of the soil, and, second, upon the amount of moisture contained 

 therein. 



The comparatively high water content of the humid East or of the 

 Great Lakes region, even during such periods of drought as occur in 

 these regions, produces a comparatively low rate of evaporation, and 

 hence a removal of water from relatively great depths in the soil. Thus, 

 after several weeks of dry weather the soil of the humid East, where 

 exposed, is probably nearly as dry as the soils of the arid West. Cam- 

 eron (1901) and Means (1901) have called attention to the occurrence 

 of true alkali spots in the East, which are evidence of such a condi- 

 tion. This subject was discussed by Hilgard (1902). The present 

 author was able to get other evidence in the same direction from the 

 soils of Northern Michigan at almost the same time that the present 

 work was begun. About June 16, 1904, some two weeks previous to the 

 beginning of the studies of desert soil, a number of soil samples were 

 collected in Kalkaska and Roscommon counties, Michigan, at a depth of 

 about 25 cm. from the surface, and the water content of these was 

 determined. The highest water content observed at this depth was 

 15 per cent by volume, in the case of a heavy clay soil covered by a 

 forest of beech, maple, elm, etc., the lighter soils ranging from 2.7 per 

 cent in the case of the sandy jackpine (Pinus Banksiana) plains to 10.3 

 per cent in the case of several loamy soils covered by Norway and white 

 pine (P. resinosa and P. strobus). Thus these soils had at that time 

 a moisture content which closely approached that of the clay of 

 Tumamoc Hill at about twice as great a depth and at the end of the 

 spring dry season. Of course it is to be remembered that, while the 

 desert soil remains at a low moisture content for many months at 

 a time, the content of the Michigan soils must often rise far above these 

 figures after the comparatively frequent rains. But the evidence is 

 clear that, with the high humidity of the latter region and the accom- 

 panying slower rate of evaporation, the soil is subjected to a more rapid 

 drying at relatively great depths than occurs in the arid regions. 



Determination was made of the comparative rates of evaporation 

 from the surfaces of several samples of clay from Tumamoc Hill with 

 different water contents. Only 100 cc. of soil were used in each case, so 

 that the experiment lacks accuracy. The samples were made up to 

 contain 10, 20, 30, and 40 per cent of moisture by volume, and were 

 placed in Stender dishes 5.5 centimeters in diameter, being tamped into 

 place as uniformly as possible. Thus the general soil surface exposed 



