Chap. III. BKOUGET UP BY WOEMS. 165 



the summer, or in winter during severe frosts. 

 If we assume that they work for only half 

 the year — though this is too low an estimate 

 — then the worms in this field would eject 

 during the year, 83*87 pounds per square 

 yard; or 18*12 tons per acre, assuming the 

 whole surface to be equally productive in 

 castings. 



In the foregoing cases some of the 

 necessary data had to be estimated, but in 

 the two following cases the results are much 

 more trustworthy. A lady, on w^hose ac- 

 curacy I can implicitly rely, offered to collect 

 during a year all the castings throwm up on 

 two separate square yards, near Leith Hill 

 Place, in Surrey. The amount collected was, 

 however, somewhat less than that originally 

 ejected by the worms ; for, as I have repeatedly 

 observed, a good deal of the finest earth is 

 washed away, whenever castings are thrown up 

 during or shortly before heavy rain. Small 

 portions also adhered to the suri'ounding 

 blades of grass, and it required too much 

 time to detach every one of them. On 

 sandy soil, as in the present instance, castings 

 are liable to crumble after dry weather, and 



