Chap. III. BEOUGHT UP BY WORMS. 169 



Leith Hill Place, during 369 days, calciilated 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. 



Tlie 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 

 were therefore broken into small particles, 

 and whilst being placed in a measure were 

 well shaken and pressed down. Those col- 

 lected on the Terrace amounted to 124" 77 

 cubic inches ; and this amount, if spread out 

 over a square yard, would make a layer 

 •09612 inch in thickness. Those collected on 

 the Common amounted to 197'56 cubic inches, 

 and would make a similar layer "1524 inch in 

 thickness. 



These thicknesses must, however, be cor- 

 rected, for the triturated castings, after being 

 well shaken down and pressed, did not make 



