average length of ten out of the above eleven castings was 2.03 inches, and half of this we may take as being one inch. It may therefore be concluded that one-third of the whole earth brought to the surface was in these cases carried down the slope through one inch.
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 hill-side with two of its sides horizontal, then it is clear that only 136 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 13 of the earth brought up can be considered to flow downwards; hence 13 of 136 or 1108 of 7.453 lbs. will cross the lower side of our square yard in a year. Now 1108 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
