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-rollecl, 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 \VATER-SfRFACE IXI'iisKD IX SHALLOW 

 TANKS, XI \R \VATI K <>K i.K'orXI) STRFACK. 



This table, the d.ita f< -r \\hich are taken from various 

 sources, exhibits clearly the enorniou- variations in evapora- 

 tion in different countries. ;md even in localities not very re- 

 mote froni each other. The lo\v evaporation near London is 

 loubtle-s due to its foggy and Iri/y 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 



