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 11 2 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 mean diameter 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.
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