Chap. III. UNDERMINED BY WORMS. 161 



round the great fragments of stone at both 

 these places, their sinking does not appear to 

 have been sensibly aided by their weight, 

 though this was considerable.* 



On the number of worms which live within 

 a given space. — We will now show, firstly, 

 what a vast number of worms live unseen by 

 us beneath our feet, and, secondly, the actual 

 weight of the earth which they bring up to 

 the surface within a given space and within 

 a given time. Hensen, who has published so 

 full and interesting an account of the habits 

 of worms,f calculates, from the number which 

 he found in a measured space, that there must 

 exist 133,000 living worms in a hectare of 

 land, or 53,767 in an acre. This latter 

 number of worms would weigh 356 pounds, 

 taking Hensen's standard of the weight of a 

 single worm, namely, three grams. It should, 

 however, be noted that this calculation is 



* Mr. R. Mallet remarks (' Quarterly Journal of Greolog. Soc.' 

 vol. xxxiii., 1877, p. 745) that " the extent to which the ground 

 beneath the foundations of ponderous architectural structures, 

 such as cathedral towers, has been known to become compressed, 

 is as remarkable as it is instructive and curious. The amount 

 of depression in some cases may be measured by feet." He 

 instances the Tower of Pisa, but adds that it was founded on 

 " dense clay." 



f * Zeitschrift fur wissensch. Zoolog.' Bd. xxviii., 1877, p. 360. 



