whole earth brought to the surface was in these cases carried down the
slope through 1 inch. {244}
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
hillside with two of its sides horizontal, then it is clear that only
1/36 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 1 3 of the earth
brought up can be considered to flow downwards; hence 1 3 of 1/36 or 1/108
of 7.453 lbs. will cross the lower side of our square yard in a year.
Now 1/108 of 7.453 lbs. is 1.1 oz. Therefore 1.1 oz. of dry earth will
annually cross each linear yard running horizontally along a slope having
the above inclination; or very nearly 7 lbs. will 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 0.2 of an inch in thickness: it therefore follows by a
calculation similar to the one already given, that 1 3 of 0.2 36, or 2.4
cubic inches of damp earth will annually cross a horizontal line one yard
in length on a hillside 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 before rain; so that the above results are thus
far exaggerated. On the other hand, during rain much of the finest earth
is washed to a considerable distance from the castings, even where the
slope is an extremely gentle one, and is thus wholly lost as far as the
above calculations are concerned. Castings ejected during dry weather
and which have set hard, lose in the same manner a considerable quantity
of fine earth. Dried castings, moreover, are apt to disintegrate into
115