256 



SOILS. 



the rapidity with which moisture will be supplied from the 

 subsoil as the surface dries. A field plowed into rough fur- 

 rows will evaporate more water than when harrowed, because 

 of the larger surface exposed ; and a harrowed field moderately 

 compacted by rolling will lose less water by evaporation than 

 when un-rolled, other things being equal. On the other hand, 

 a thoroughly compacted surface, even if suffering less loss at 

 first than a plowed or harrowed field, will continue to lose 

 moisture longer by withdrawing it from the substrata by its 

 superior capillary suction; while a loose surface, once dried 

 out, will prevent farther loss from the subsoil very effectually, 

 as stated below. 



TABLE SHOWING EVAPORATION, FROM WATER-SURFACE EXPOSED IN SHALLOW 

 TANKS, NEAR WATER OR GROUND SURFACE. 



This table, the data for which are taken from various 

 sources, exhibits clearly the enormous variations in evapora- 

 tion in different countries, and even in localities not very re- 

 mote from each other. The low evaporation near London is 

 doubtless due to its foggy and hazy atmosphere, but it is not 

 clear why Rothamsted should show so low an evaporation 

 compared with Oxford. Tropical Demerara stands nearest to 

 Oxford in its evaporation ; Bombay indicates its location on 



