122 



THE OCEAN. 



which insure a freer and deeper channel for navigation, are those 

 which prevent the injuries caused by the great violence of the 



Fis. 44.— Plan of the " mascarct " or tidal wave observed In the narrows of the Seine (after M. Parfclot). 



tidal waves.* The mascarct of the Seine disappeared recently for 

 some years, owing to the elevation of a bank of sand, like a dike 

 which prevented the entrance of the tide into the bed of the river. 

 The encounter of the mascarct and the fluvial current have again 



Fig. 45.— Plan of two tidal waves crossinj? each other's course on the banks of the bay of the Seine 



fafter M. Partiot). 



raised this bank of sand at a little distance. On striking against this 

 new obstacle, the tidal wave rises up to surmount it. Diiferent 

 hydraulic works, undertaken in the beds of the Garonne and the 

 Dordogne above the Bec-d'Ambez, have also often modified the 

 phenomena of the mascarct there. 



The sudden appearance of the tide in estuaries raises the fluvial 

 waters very rapidly from the level of low to that of high water. At 

 * Partiot, Annales des Fonts et Chaussees, t. i. 1861. 



