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 whichhave set hard, lose in the same manner a considerable quantity of fine earth.Dried castings, moreover, are apt to disintegrate into little pellets, which often roll or are blown down any inclined surface.Therefore the above result, namely, that 24 cubic inches of earth (weighing 1.85 oz.whilst damp) annually crosses a yard- line of the specified kind, is probably not much if at all exaggerated.
This amount is small; but we should bear in mind how many branching valleys intersect most countries, the whole length of which must be very great; and that earth is steadily travelling down both turf- covered sides of each valley.For every 100 yards in length in a valley with sides sloping as in the foregoing cases, 480 cubic inches of damp earth, weighing above 23 pounds, will annually reach the bottom.Here a thick bed of alluvium will accumulate, ready to be washed away in the course of centuries, as the stream in the middle meanders from side to side.
If it could be shown that worms generally excavate their burrows at right angles to an inclined surface, and this would be their shortest course for bringing up earth from beneath, then as the old burrows collapsed from the weight of the superincumbent soil, the collapsing would inevitably cause the whole bed of vegetable mould to sink or slide slowly down the inclined surface.But to ascertain the direction of many burrows was found too difficult and troublesome.A straight piece of wire was, however, pushed into twenty-five burrows on several sloping fields, and in eight cases the burrows were nearly at right angles to the slope; whilst in the remaining cases they were indifferently directed at various angles, either upwards or downwards with respect to the slope.
In countries where the rain is very heavy, as in the tropics, the castings appear, as might have been expected, to be washed down in a greater degree than in England.Mr.Scott informs me that near Calcutta the tall columnar castings (previously described), the diameter of which is usually between 1 and 1.5 inch, subside on a level surface, after heavy rain, into almost circular, thin, flat discs, between 3 and 4 and sometimes 5 inches in diameter.Three fresh castings, which had been ejected in the Botanic Gardens "on a slightly inclined, grass-covered, artificial bank of loamy clay," were carefully measured, and had a mean height of 2.17, and a meandiameter of 1.43 inches; these after heavy rain, formed elongated patches of earth, with a mean length in the direction of the slope of 5.83 inches.As the earth had spread very little up the slope, a large part, judging from the original diameter of these castings, must have flowed bodily downwards about 4 inches.Moreover some of the finest earth of which they were composed must have been washed completely away to a still greater distance.In drier sites near Calcutta, a species of worm ejects its castings, not in vermiform masses, but in little pellets of varying sizes: these are very numerous in some places, and Mr.Scott says that they "are washed away by every shower."I was led to believe that a considerable quantity of fine earth is washed quite away from castings during rain, from the surfaces of old ones being often studded with coarse particles.Accordingly a little fine precipitated chalk, moistened with saliva or gum-water, so as to be slightly viscid and of the same consistence as a fresh casting, was placed on the summits of several castings and gently mixed with them.These castings were then watered through a very fine rose, the drops from which were closer together than those of rain, but not nearly so large as those in a thunderstorm; nor did they strike the ground with nearly so much force as drops during heavy rain.A casting thus treated subsided with surprising slowness, owing as I suppose to its viscidity.It did not flow bodily down the grass-covered surface of the lawn, which was here inclined at an angle of 16 degrees 20 seconds; nevertheless many particles of the chalk were found three inches below the casting.The experiment was repeated on three other castings on different parts of the lawn, which sloped at 2 degrees 30 seconds, 3 degrees and 6 degrees; and particles of chalk could be seen between 4 and 5 inches below the casting; and after the surface had become dry, particles were found in two cases at a distance of 5 and 6 inches.Several other castings with precipitated chalk placed on their summits were left to the natural action of the rain.In one case, after rain which was not heavy, the casting was longitudinally streaked with white.In two other cases the surface of the ground was rendered somewhat white for a distance of one inch from the casting; and some soil collected at a distance of 2.5 inches, where the slope was 7 degrees, effervesced slightlywhen placed in acid.After one or two weeks, the chalk was wholly or almost wholly washed away from all the castings on which it had been placed, and these had recovered their natural colour.
It may be here remarked that after very heavy rain shallow pools may be seen on level or nearly level fields, where the soil is not very porous, and the water in them is often slightly muddy; when such little pools have dried, the leaves and blades of grass at their bottoms are generally coated with a thin layer of mud.This mud I believe is derived in large part from recently ejected castings.