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half of this we may take as being 1 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 1 inch。 ｛77｝
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 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。2x36； 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
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 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。 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 cas