On Sand-hanks. 115 



are very little compared with the imports. That the cause 

 now assigned is the true one is confirmed by the fact already 

 mentioned, that no diminution in the height of the shoal be- 

 tween the Mount and Marazion had taken place for 220 

 years after the time of Leland, during which period there was 

 but little agriculture or commerce in the bay. 



Thus the sands, which, in some neighbourhoods, accumu- 

 late in the sea to the great peril of mariners, have here been 

 deposited on the shore, where for centuries they have served 

 as pastui-e grounds or pleasant walks, and as barriers against 

 the sea, but have latterly proved still more valuable for agri- 

 cultural purposes. Husbandmen are aware that the soil from 

 one district on being distributed over another proves often- 

 times highly beneficial to the latter. When, for example, 

 fragments of granite are washed down into the bay, reduced to 

 sand by the action of the waves, and then spread over a kil- 

 las stratum, they tend greatly to its fertilization, and vice 

 versa. Thus, in the great geological laboratory, during the 

 lapse of ages, manures of various kinds — calcareous, siliceous, 

 and argillaceous — have been prepared, without the least ex- 

 penditure of human labour, and are treasured up on our 

 coasts to be used on the neighbouring farms in any quantities 

 that may be required. 



On the Internal Pressure to which Hock Masses may be sub- 

 jected, and its possible influence in the Production of the La- 

 minated Structure. By W. HOPKINS, M.A., F.E..S.* 



If a plane, of indefinitely small extent, pass through any 

 proposed point in the interior of a continuous solid mass in 

 a state of constraint, the resultant pressure or tension on this 

 plane will vary with the angular position of the plane, and its 

 direction will not, as in fluid masses, be generally perpendi- 

 cular to the plane. There are, however, three angular posi- 

 tions in which the direction of the pressure does coincide 

 with a perpendicular to the plane. These are called princi- 



* Pri)ceedings of the Cambridge PhiloB )phic vl Society, AFay 3, 1847. 



