FOR GREA T BRITAIN AND IRELAND. 103 



SHOALING OF NATURAL BAYS. 



It is not a matter of course, that when a space of water is 

 enclosed with solid piers, a great deposit should follow 

 although such is undeniably sometimes the case. 



"The shoaling of channels," says Mr. Stevenson, 

 " which have been dredged deeper than the original 

 bottom, forms no proper ground for predicting a similar 

 reduction of depth due to the natural profile of conservancy 

 of the shore, which generally preserves its symmetry with 

 remarkable persistency, even within artificial enclosing 

 walls." 



CAUSE OF ENCLOSED HARBOURS KEEPING OPEN. 



" Mr. Calver, who is strongly opposed to close harbours, 

 on the ground that they will fill up, makes an exception 

 regarding small tidal harbours, which he says are kept clear 

 by the ' scavenging process ' of high winds during ebbing 

 tide, and that the most ' diminutive lipper ' is effective in 

 moving the lighter kinds of deposit. The surface-ripple 

 described by Mr. Calver will no doubt have a certain effect." 

 Mr. Stevenson considers there must be some more efficient 

 cause to keep ports and harbours open, and attributes it 

 to the run, for wherever there is a ground-swell or run, 

 ordinary waves produced by a gale are sufficient agents, he 

 thinks, to perform this office. 



In gales the undulations, as he observes, " become waves of 

 translation, and as each wave as it enters the basin brings a 

 certain amount of water into it, it ultimately must escape 

 seawards, or the water would stand at a higher level in the 

 harbour than the sea outside. The under-surface current 

 thus produced runs probably very near the bottom. 



