Chap. III. BEOUGHT UP BY WORMS. 171 



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. 



(I.) 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 45 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 uoi- 

 formly over a square yard. The dry castings 



