CHAP. VI. AIDED BY WORMS. 273 



length in a valley with sides sloping as in the 

 foregoing cases, 480 cubic inches of damp 

 earth, weighing above 23 pounds, will annu- 

 ally reach the bottom. Here a thick bed of 

 alluvium will accumulate, ready to be washed 

 away in the course of centuries, as the stream 

 in the middle meanders from side to side. 



If it could be shown that worms generally 

 excavate their burrows at right angles to 

 an inclined surface, and this would be 

 their shortest course for bringing up earth 

 from beneath, then as the old burrows col- 

 lapsed from the weight of the superincum- 

 bent soil, the collapsing would inevitably 

 cause the whole bed of vegetable mould to 

 sink or slide slowly down the inclined sur- 

 face. But to ascertain the direction of many 

 burrows was found too difficult and trouble- 

 some. A straight piece of wire was, however, 

 pushed into twenty-five burrows on several 

 sloping fields, and in eight cases the burrows 

 were nearly at right angles to the slope ; whilst 

 in the remaining cases they were indifferently 

 directed at various angles, either upwards or 

 downwards with respect to the slope. 



In countries where the rain is very heavy, 



