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Show 172 TIIICKNESS OF TilE MOULD UrrAP. III. of this chapter), we will give the following sunnnary :- SUllll\fARY OF TilE THICKNESS OF THE Mour,D ACCUMULA'I'ED OVER 013JI~C'l'S LEFT S'l'HEWED ON TilE SURFACE, IN 'flm COURSE OF 'l'EN YEA HS. rrhe accumulation of mould tl.udng 14 J years on the surface of a dry, sandy, grass-field 110ar Macr Ilall, amounLed to 2 · 2 iuchcs in 10 years. 'l'he accumulation during 21} years on a swampy field ncar Maer Dall, amoun Led to nearly 1 · 9 inch iu 10 years. The accumulation during 7 years on a very swampy field ncar Mael' llall a moun tcrl to 2 ·1 inches in 10 years. The accumulation during 20 years, on good, argillaceous pasture-land over the Chalk at Down, amounted to 2 · 2 inches in 10 years. 'l'he accumulation during 30 years on the side of a valley over the Chalk at Down, the soil being argillaceous, very poor, and only just converLed into pasture (so Lhat it was for some years unfavourable for worms), amounted to 0·83 incbel:l iu 10 ycarl:l. In these cases (excepting the last) it may be seen that the amount of earth brought to the surface during 10 years is somewhat greater than that calculated frorr1 the castings which were actually weighed. This excess may be partly accounted for by the loss which the weighed castings had previou ly undergone through being washed by rain, by the adhesion of particles to the blades of the surrounding grass, and by their crumbling when dry. Nor rnust we overlook other agencies CrrAP. HI. ANNUALLY AUCUMULA'l'ED. 173 which in all ordinary cases add to the amount of mould, and which would not be included in the castings that were collected namely, the fine earth brought up to the' s~rface by burrowing larvro and insects, especrally by ants. The earth brought up by moles generally has a somewhat different appearance from vegetable mould; but after a time would n~t be di tinguishable from it. In dry countnes, moreover, the wind plays an important part in carrying dust from one place to another, and· even in England it must add to the mould on fields near great roads. But in our country these latter several agencies appear to be of quito suLordinate importance in comparison with the action of worms. We have no means of judging how great a wei~ht of earth a single full-sized worm ejects dunng a year. I-Iensen estimates that 53, 7G7 worms exist in an acre of land ; but this is founded on the number found in gardens, and he believes that only about half as many live in corn-fields. I-Iow rnany live in old pasture land js unknown; but if we assume that half the above number, or 26,886 worms live on such land, then taking from the previous |
