Chap. VI. AIDED BY WOKMS. 269 



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-third 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 



