270 DENUDATION OF THE LAND CHAP. VI. 



average length of ten out of the ahove eleven 

 castings was 2*03 inches, and half of this we 

 may take as heing 1 inch. It may therefore 

 be concluded that one-third of the whole 

 earth brought to the surface was in these 

 cases carried down the slope through 1 inch.* 

 It was shown in the third chapter that on 

 Leith Hill Common, dry earth weighing at 

 least 7'453 Ibs. was brought up by worms to 

 the surface on a square yard in the course of 

 a year. If a square yard be drawn on a 

 hill-side with two of its sides horizontal, then 

 it is clear that only -^g- part of the earth 

 brought up on that square yard would be 

 near enough to its lower side to cross it, 

 supposing the displacement of the earth to 

 be through one inch. But it appears that 

 only ^ of the earth brought up can be con- 

 sidered to flow downwards ; hence -g- of -^ or 

 y^-g- of V'453 Ibs. will cross the lower side of 

 our square yard in a year. Now -j-J^ of 

 7'453 Ibs. is I'l oz. Therefore I'l oz. of dry 

 earth will annually cross each linear yard run- 



* Mr James Wallace Has pointed out that it is necessary to 

 take into consideration the possibility of burrows being made at 

 right angles to the surface instead of vertically down, in which 

 case the lateral displacement of the soil would be increased. 



