THE BAY OF BISCAY. 167 



It occurs in all seas, but the disposition of the coast- 

 line influences its intensity. It is by this ground 

 swell and not by the superficial waves that boulders, 

 pebbles, sand, and all submerged objects are thrown 

 on the shore, and this is also the cause of those 

 breakers occurring above submarine shoals, which 

 are so much dreaded by sailors, and which render 

 many channels impracticable even during the calmest 

 weather, as we see amongst other places at the 

 entrance to the bay of Saint-Jean-de-Luz. To the 

 same kind of ground swell we must also ascribe that 



O 



perpetual tempest which seems to prevail at the bar 

 of the Adour, and at several other points of this 

 coast. Colonel Emy also refers to it the singular 

 phenomenon which I have observed on a small scale 

 in the river of St. Sebastian, but which is much 

 more fully developed in almost all large rivers. It 

 is known to the English by the name of the bore, to 

 the bargemen of the Seine as la barre, to those of 

 the Dordogne as le mascaret, and to the boatmen of 

 the Amazon as la pororoca. At the mouth of the 

 latter river, during spring tides at the periods of 

 new and full moon, the sea, instead of continuing to 

 rise for six hours, attains its greatest elevation in 



the theory of Colonel Emy in relation to the formation of the ground 

 swell, and he regards it as owing to the action of waves, which are 

 formed not by simple undulations, but by a real transportation of 

 liquid. All the effects attributed to the ground swell are ascribed 

 by him to the back stroke. 



The views which we have here expressed correspond very nearly 

 with those of M. de Caligny, although we agree with Colonel Emy 

 in attributing a very decided influence to submarine rocks in the 

 formation of the ground swell. 



U 4 



