Chap. VI. AIDED BY WOKMS. 267 



average length of ten out of the above 

 eleven castings was 2*03 inches, and half of 

 this we may take as being one 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 

 one inch. 



It was shown in the third chapter that on 

 Leith Hill Common, dry earth weighing at 

 least 7*453 lbs. 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 -^ 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 -J- of -g^ or 

 -j-g^g- of 7*453 lbs. will cross the lower side of 

 our square yard in a year. 'Now y^-g- of 

 7*453 lbs. is 1*1 oz. Therefore I'l oz. of dry 

 earth will annually cross each linear yard run- 

 ning horizontally along a slope having the 

 above inclination ; or very nearly 7 lbs. will 



