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 therefore follows by a calculation similar to the one already given, that 13 of .2 × 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 length on our inclined surface.
In these calculations it has been assumed that the castings flow a short distance downwards during the whole year, but this occurs only with those ejected during or shortly
