OCR Text |
Show 170 'l'IIICKNESS OF 'l'IIE MOULD CHAP. III. were therefore broken into small particles, and whilst being placed in a measure were well shaken and pressed down. Tho e collected on the Terr~ce amounted to 124·77 cubic inches; and this arnount, if spread out over a square yard, would make a layer ·08612 inch in thickness. Those collected on the Common amounted to 197·56 cubic inches, aud would make a similar layer ·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 co1npact. Yet mould is far from being compact, as is shown by the number of airbubbles 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 tl1in oblong block of somewhat argillaceous mould (with the turf pared off) was measured before being broken up, was well dried and CHAP. HI. ANNUALLY ACCUMULA'l'ED. 171 again 1neasured. The drying caused it to sl1rink by i-of its original bulk, judging fron1 exterior measurements alone. It was then triturated and partly reduced to powder, in tho same manner as the castings had been treated, and its lmlk now exceeded (notwithstanding shrinlm()'e from drying) by ft that of the original block of damp mould. Therefore the above calculated thickness of the layer, formed by tho castings from the Terrace, after being damped and spread over a square yard, would have to be reduced by -(6 ; and this will reduce the layer to ·09 of an inch, so that a layer ·8 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 ·1428 inch, or in the course of 10 years 1·L!28 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 nearly 1! inch in 10 years. In order to compare these r suits with those deduced from the rates at which small objects left on the surfaces of gra ·s-fie]ul:) becmne buried (as descriLed in the early part |