EFFECTS OF THE TIDES ON EIYEES. 



123 



Tancarville, whicli is the precise spot where the Seine discharges 

 itself into the bay, and where the tide exceeds a mean amplitude of 

 about 13 feet, the entire rising of the waters is accomplished in two 

 hours, while the fall of the liquid mass, driven back by the tide, 

 occupies about 10 hours. The river having to discharge during the 

 period of ebb not only that which the flow had brought to it, but also 

 the fresh waters from higher up, must follow its normal course 

 towards the sea during a space of time longer than that in which it 

 is driven back by the rising tide. For each point of the river-bed 

 the duration of the flow is generally the shorter the farther that 

 point is from the sea : the force of the tide is gradually exhausted, 

 and towards the end of its course it only momentarily retards the 

 speed of the fluvial current. 



The amplitude of the tides diminishes, likewise, in proportion to 

 their progress up the stream in rivers. The mass of fresh water 



9 -jj cj 

 H ** <^ 





Fig. 46.— Tides of the Garonne. 



flowing incessantly within the channel prevents the low tide from 

 sinking, as it does on the sea-shore; and as to the high tide, its 

 shorter duration does not allow it to rise to a much higher level than 

 that which it attains on the strands and clifls by the ocean. Thus, 

 in the Garonne, the diflerence between the ebb and flow diminishes 

 gradually above the Bec-d'Ambez, and near Castets, at about 95 

 miles from the sea, it is finally reduced to zero. In certain places, 

 it is true, particular circumstances may cause apparent exceptions to 

 this general law ; a promontory rising before the tidal wave like that 

 of Tancarville in the bay of the Seine, bars the way to the marine 

 waters, and gives them in consequence a greater relative height above 

 low water. But in spite of these abrupt projections, the mean ampli- 

 tude of the tide diminishes from the lower to the upper course, and 

 finally it becomes imperceptible. 



