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. 



(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 uni- 

 formly over a square yard. The dry castings 



