按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
for safety's sake within a year； 12 oz。 of castings were thrown up
on this one square foot； or 6。75 pounds on the square yard； and
this would give 14。58 tons per acre。
In a field at the bottom of a valley in the chalk （see No。 2 in the
foregoing table）； a square yard was measured at a spot where very
large castings abounded； they appeared； however； almost equally
numerous in a few other places。 These castings； which retained
perfectly their vermiform shape； were collected； and they weighed
when partially dried； 1 lb。 13。5 oz。 This field had been rolled
with a heavy agricultural roller fifty…two days before； and this
would certainly have flattened every single casting on the land。
The weather had been very dry for two or three weeks before the day
of collection； so that not one casting appeared fresh or had been
recently ejected。 We may therefore assume that those which were
weighed had been ejected within； we will say； forty days from the
time when the field was rolled；that is； twelve days short of the
whole intervening period。 I had examined the same part of the
field shortly before it was rolled； and it then abounded with fresh
castings。 Worms do not work in dry weather during the summer； or
in winter during severe frosts。 If we assume that they work for
only half the yearthough this is too low an estimatethen the
worms in this field would eject during the year； 8。387 pounds per
square yard； or 18。12 tons per acre； assuming the whole surface to
be equally productive in castings。
In the foregoing cases some of the necessary data had to be
estimated； but in the two following cases the results are much more
trustworthy。 A lady； on whose accuracy I can implicitly rely；
offered to collect during a year all the castings thrown up on two
separate square yards； near Leith Hill Place； in Surrey。 The
amount collected was； however； somewhat less than that originally
ejected by the worms； for； as I have repeatedly observed； a good
deal of the finest earth is washed away； whenever castings are
thrown up during or shortly before heavy rain。 Small portions also
adhered to the surrounding blades of grass； and it required too
much time to detach every one of them。
On sandy soil； as in the present instance； castings are liable to
crumble after dry weather； and particles were thus often lost。 The
lady also occasionally left home for a week or two； and at such
times the castings must have suffered still greater loss from
exposure to the weather。 These losses were； however； compensated
to some extent by the collections having been made on one of the
squares for four days； and on the other square for two days more
than the year。
A space was selected （October 9th； 1870） for one of the squares on
a broad； grass…covered terrace； which had been mowed and swept
during many years。 It faced the south； but was shaded during part
of the day by trees。 It had been formed at least a century ago by
a great accumulation of small and large fragments of sandstone；
together with some sandy earth； rammed down level。 It is probable
that it was at first protected by being covered with turf。 This
terrace； judging from the number of castings on it； was rather
unfavourable for the existence of worms； in comparison with the
neighbouring fields and an upper terrace。 It was indeed surprising
that as many worms could live here as were seen； for on digging a
hole in this terrace； the black vegetable mould together with the
turf was only four inches in thickness； beneath which lay the level
surface of light…coloured sandy soil； with many fragments of
sandstone。 Before any castings were collected all the previously
existing ones were carefully removed。 The last day's collection
was on October 14th； 1871。 The castings were then well dried
before a fire； and they weighed exactly 3。5 lbs。 This would give
for an acre of similar land 7。56 tons of dry earth annually ejected
by worms。
The second square was marked on unenclosed common land； at a height
of about 700 ft。 above the sea； at some little distance from Leith
Hill Tower。 The surface was clothed with short； fine turf； and had
never been disturbed by the hand of man。 The spot selected
appeared neither particularly favourable nor the reverse for worms；
but I have often noticed that castings are especially abundant on
common land； and this may； perhaps； be attributed to the poorness
of the soil。 The vegetable mould was here between three and four
inches in thickness。 As this spot was at some distance from the
house where the lady lived； the castings were not collected at such
short intervals of time as those on the terrace； consequently the
loss of fine earth during rainy weather must have been greater in
this than in the last case。 The castings moreover were more sandy；
and in collecting them during dry weather they sometimes crumbled
into dust； and much was thus lost。 Therefore it is certain that
the worms brought up to the surface considerably more earth than
that which was collected。 The last collection was made on October
27th； 1871； i。e。； 367 days after the square had been marked out and
the surface cleared of all pre…existing castings。 The collected
castings； after being well dried； weighed 7。453 pounds； and this
would give； for an acre of the same kind of land； 16。1 tons of
annually ejected dry earth。
SUMMARY OF THE FOUR FOREGOING CASES。
（1。） Castings ejected near Nice within about a year； collected by
Dr。 King on a square foot of surface； calculated to yield per acre
14。58 tons。
（2。） Castings ejected during about 40 days on a square yard； in a
field of poor pasture at the bottom of a large valley in the Chalk；
calculated to yield annually per acre 18。12 tons。
（3。） Castings collected from a square yard on an old terrace at
Leith Hill Place； during 369 days； calculated to yield annually per
acre 7。56 tons。
（4。） Castings collected from a square yard on Leith Hill Common
during 367 days； calculated to yield annually per acre 16。1 tons。
The thickness of the layer of mould； which castings ejected during
a year would form if uniformly spread out。As we know； from the
two last cases in the above summary； the weight of the dried
castings ejected by worms during a year on a square yard of
surface； I wished to learn how thick a layer of ordinary mould this
amount would form if spread uniformly over a square yard。 The dry
castings were therefore broken into small particles； and whilst
being placed in a measure were well shaken and pressed down。 Those
collected on the Terrace amounted to 124。77 cubic inches； and this
amount； if spread out over a square yard； would make a layer 0。9627
inch in thickness。 Those collected on the Common amounted to
197。56 cubic inches； and would make a similar layer 0。1524 inch in
thickness；
These thicknesses must； however； be corrected； for the triturated
castings； after being well shaken down and pressed； did not make
nearly so compact a mass as vegetable mould； though each separate
particle was very compact。 Yet mould is far from