Chap. HI. ANNUALLY ACCUMULATED. 173 



on fields near great roads. But in our county 

 these latter several agencies appear to be of 

 quite subordinate importance in comparison 

 with the action of worms. 



We have no means of judging how great a 

 weight of earth a single full-sized worm ejects 

 during a year. Hensen estimates that 53,767 

 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. How many live in old pasture 

 land is unknown ; but if we assume that half 

 the above number, or 26,886 worms live on 

 such land, then taking from the previous 

 summary 15 tons as the weight of the castings 

 annually thrown up on an acre of land, each 

 worm must annually eject 20 ounces. A full- 

 sized casting at the mouth of a single burrow 

 often exceeds, as we have seen, an ounce in 

 weight ; and it is probable that worms eject 

 more than 20 full-sized castings during a 

 year. If they eject annually more than 20 

 ounces, we may infer that the worms which 

 live in an acre of pasture land must be less 

 thnn 26,886 in number. 



Worms live chiefly in the superficial mould, 



