266 DENUDATION OP THE LAND Chap. VL 



castings on the same lawn where the mean 

 slope was 12° 5'. 



We may, however, take the above eleven 

 cases, which are accurate as far as they go, 

 and calculate the weight of the ejected earth 

 which annually flows down a slope having a 

 mean inclination of 9° 26'. This was done 

 by my son George. It has been shown 

 that almost exactly two-thirds of the ejected 

 earth is found below the mouth of the 

 burrow and one-third above it. Now if the 

 two-thirds which is below the hole be divided 

 into two equal parts, the upper half of this 

 two-thirds exactly counterbalances the one- 

 third which is above the hole, so that as far 

 as regards the one-thiid above and the upper 

 half of the two-thirds below, there is no flow 

 of earth down the hill-side. The earth con- 

 stituting the lower half of the two-thirds is, 

 however, displaced through distances which 

 are different for • every part of it, but which 

 may be represented by the distance between 

 the middle point of the lower half of the 

 two-thirds and .the hole. So that the average 

 distance of displacement is a half of the 

 whole length of the worm -casting. Now the 



