TIDES. 51 



which, where steep gradients occur owing to difference in water- 

 level, often runs with great velocity until equilibrium is re-estab- 

 lished. The rapid tidal currents through the Pentland Frith, 

 and through many of the narrow channels to be found about 

 the Hebrides and elsewhere, are produced in this manner. 



The velocity at which these tidal currents run is, however, 

 quite distinct from that of the tidal undulation. Thus in the 

 North Sea, along the east coast of Scotland, the velocity of the 

 tidal stream is from 2 to 3 knots per hour, while that of the 

 tidal undulation is upwards of 50 knots. 



When tidal water passes into converging river channels, or 

 estuaries, its momentum causes it to heap up, and abnormal 

 range of tide is the result. A shoaling, or gradual decrease in 

 the depth of the water, produces a similar effect, the velocity 

 of the tidal wave being reduced, and its height increased. 



The gradual convergence of the shores, and shoaling of the 

 water in the Bay of Fundy, give rise to the exceptional and 

 notorious range of tide there, which is said to amount to fully 

 GO feet at the head of the bay. 



When it is considered that the range of a spring tide at the 

 mouth of the bay is only about 8 feet, such an extraordinary 

 increase is certainly remarkable. 



This bay, which lies between New Brunswick and Nova 

 Scotia, is roughly 180 miles in length. Its width at the entrance 

 which approximates in form to a bell-mouth is about 60 

 miles, converging gradually to about 20 miles near the head of 

 the bay, where it bifurcates into two comparatively narrow and 

 shoal arms or inlets. 



We find the tidal range affected in precisely the same manner 

 in all estuaries and bays where the tide is free to enter, only in 

 varying degree according to their several physical conditions. The 

 estuary of the Bristol Channel may be cited as a good example. 

 There the range of tide at the entrance is about 18 feet; but, 

 owing to the funnel-shaped contraction of the channel, and 

 gradual shoaling of the water, it is made to rise to about 27 

 feet at Swansea, 37 feet at Cardiff, and 50 feet at Chepstow. 



At Avranches, where the range of tide at springs is about 39 

 feet, the cause, though somewhat similar, is not quite the same. 

 A large portion of the tidal wave, in passing up the English 

 Channel, is stopped by the French coast (Cape La Hogue) pro- 

 jecting so far across the English Channel, opposite to St. Alban's 



