IV.] WATER LIFTED BY SURFACE TENSION 99 



cases the amounts were probably less than would be 

 realised under field conditions, because the evapora- 

 tion was not enough to dry the surface, and was 

 further checked by the formation of a saline crust on 

 the surface. Working in the field, King obtained a 

 daily loss at the evaporating surface of 1-3 lb. per 

 square foot, or 0-19 inch of water, the water table 

 being from 4 to 5 feet below the surface. 



The relative powers of different soils to lift water by 

 capillarity alone is well seen during any long summer 

 drought, such as prevailed in the south of England 

 during 1899 and 1900. In the Thames valley, fields of 

 swedes grew till the roots were one or two inches in 

 diameter, and then died outright, although the water 

 table was not more than 16 or 20 feet below; yet the 

 coarse-grained gravel of which the subsoil was composed 

 could not lift the water in any appreciable quantity to 

 the surface. In the same seasons the crops upon the 

 chalk hills were quietly growing ; though the water table 

 was as much as 200 feet below the surface, there was 

 still a steady capillary rise of water through the fine- 

 grained chalk. In a drought it is always the gravels 

 and coarse sands which suffer first, and this not because 

 they start with less water, for we have already seen 

 that what they absorb they can give up almost wholly 

 to the plant, whereas a clay, which absorbs much more, 

 can only hand over about the same proportion to the 

 plant as the sand did, so much being held as hygroscopic 

 moisture. The plant suffers because the small surface 

 of the soil particles gives the coarse-grained sand or 

 gravel a very limited power of lifting the subsoil water 

 to the roots of the plant. Should a drought continue, 

 the clay soils begin to suffer next, for though they start 

 with large supplies of water and have an extensive sur- 

 face of soil particles, yet water can be moved so slowly 



