164 WEIGHT OF EABTH Chap. JIL 



yard; and this would give 14*58 tons per 

 acre. 



In a field at the bottom of a valley in the 

 chalk (see No. 2 in the foregoing table), a 

 square yard was measured at a spot where 

 very large castings abounded ; they appeared, 

 however, almost equally numerous in a few 

 other places. These castings, which retained 

 perfectly their vermiform shape, were col- 

 lected ; and they weighed when partially 

 dried, 1 lb. 13^ oz. This field had been 

 rolled with a heavy agricultural roller fifty-two 

 days before, and this would certainly have 

 flattened every single casting on the land. 

 The weather had been very dry for two or 

 three weeks before the day of collection, so 

 that not one casting appeared fresh or had 

 been recently ejected. We may therefore 

 assume that those which were weighed had 

 been ejected within, we will say, forty days 

 from the time when the field was rolled, — 

 that is, twelve days short of the whole inter- 

 vening period. I had examined the same 

 part of the field shortly before it was rolled, 

 and it then abounded with fresh castings. 

 Wonns do not work in dry weather dming 



