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 being compact, as is shown by the
number of air-bubbles which rise up when the surface is flooded with
water. It is moreover penetrated by many fine roots. To ascertain
approximately by how much ordinary vegetable mould would be increased in
bulk by being broken up into small particles and then dried, a thin
oblong block of somewhat argillaceous mould (with the turf pared off) was
measured before being broken up, was well dried and again measured. The
drying caused it to shrink by 1/7 of its original bulk, judging from
exterior measurements alone. It was then triturated and partly reduced
to powder, in the same manner as the castings had been treated, and its
bulk now exceeded (notwithstanding shrinkage from drying) by 1/16 that of
the original block of damp mould. Therefore the above calculated
thickness of the layer, formed by the castings from the Terrace, after
being damped and spread over a square yard, would have to be reduced by
1/16; and this will reduce the layer to 0.09 of an inch, so that a layer
0.9 inch in thickness would be formed in the course of ten years. On the
same principle the castings from the Common would make in the course of a
single year a layer 0.1429 inch, or in the course of 10 years 1.429 inch,
in thickness. We may say in round numbers that the thickness in the
former case would amount to nearly 1 inch, and in the second case to
75