268 DENUDATION OF THE LAND Chap. VL 



annually cross a horizontal line, 100 yards in 

 length, on a hill-side having this inclination. 



A more accurate, though still very rough, 

 calculation can be made of the bulk of earth, 

 which in its natural damp state annually 

 flows down the same slope over a yard-line 

 drawn horizontally across it. From the 

 several cases given in the third chapter, it 

 is known that the castings annually brought 

 to the surface on a square yard, if uniformly 

 spread out would form a layer "2 of an inch 

 in thickness : it therefoie follows by a 

 calculation similar to the one already given, 

 that -^ of '2 X 36, or 2*4 cubic inches of damp 

 earth will annually cross a horizontal line one 

 yard in length on a hill-side with the above 

 inclination. This bulk of damp castings 

 was found to weigh 1'85 oz. Therefore 

 11"56 lbs. of damp earth, instead of 7 lbs. of 

 dry earth as by the former calculation, would 

 annually cross a line 100 yards in bngth on 

 our inclined surface. 



In these calculations it has been assumed 

 that the castings flow a short distance down- 

 wards during the whole year, but this occurs 

 only with those ejected during or shortly 



