measurements within about 1 8 of an inch, but one of the castings was too
irregular to admit of measurement. The average length in the direction
of the slope of the remaining ten castings was 2.03 inches. The
castings were then divided with a knife into two parts along a
horizontal line passing through the mouth of the burrow, which was
discovered by slicing off the turf; and all the ejected earth was
separately collected, namely, the part above the hole and the part
below. Afterwards these two parts were weighed. In every case there was
much more earth below than above; the mean weight of that above being
103 grains, and of that below 205 grains; so that the latter was very
nearly double the former. As on level ground castings are commonly
thrown up almost equally round the mouths of the burrows, this
difference in weight indicates the amount of ejected earth which had
flowed down the slope. But very many more observations would be
requisite to arrive at any general result; for the nature of the
vegetation and other accidental circumstances, such as the heaviness of
the rain, the direction and force of the wind, &c., appear to be more
important in determining the quantity of the earth which flows down a
slope than its angle. Thus with four castings on my lawn (included in
the above eleven) where the mean slope was 7 19 , the difference in
the amount of earth above and below the burrows was greater than with
three other castings on the same lawn where the mean slope was 12 5 .
We may, however, take the above eleven cases, which are accurate as far
as they go, and calculate the weight of the ejected earth which annually
flows down a slope having a mean inclination of 9 26 . This was done by
my son George. It has been shown that almost exactly two-thirds of the
ejected earth is found below the mouth of the burrow and one-third above
it. Now if the two-thirds which is below the hole be divided into two
equal parts, the upper half of this two-thirds exactly counterbalances
the one-third which is above the hole, so that as far as regards the
one-third above and the upper half of the two-thirds below, there is no
flow of earth down the hill-side. The earth constituting the lower half
of the two-thirds is, however, displaced through distances which are
different for every part of it, but which may be represented by the
distance between the middle point of the lower half of the two-thirds and
the hole. So that the average distance of displacement is a half of the
whole length of the worm-casting. Now the average length of ten out of
the above eleven castings was 2.03 inches, and half of this we may take
as being 1 inch. It may therefore be concluded that one-third of the
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