32 SEA AXD LAND 



l-urthcrmore, they are more or less affected by the action 

 of the wind ; a heavy storm bU)wing off the shore will cause 

 the tide to retreat farther and advance less far than when the 

 wind is l)lo\viniX violently toward the coast. These varying 

 conditions much affect the action of the tidal waves on most 

 coast-lines. 



The form of every coast to which the tides find access very 

 greatly affects the way in which they operate upon it. In the 

 open sea the rise and fall of the tide is slight, probably 

 not exceeding a foot or fifteen inches. If the shores of the 

 continents were straight shore walls parallel to each other, 

 with the sea very deep at their bases, the tidal swing would 

 l)e no o-reater than it is in the middle of the great Southern 

 Ocean ; but, as we know, the coast abounds in re-entrant and 

 salient angles, deep bays, and strong promontories, and in this 

 complication of paths which they open to the waters the tide 

 is curiously affected. Wherever an ocean or bay opens a 

 wide mouth to the entering tide and narrows its shores at the 

 head of the re-entrant, the swift-running broad wave moving 

 inward, usually at the rate of several hundred miles an hour, 

 is compressed in the narrowing channel and forced to rise to 

 a greater height than in the open sea. Thus in the North 

 Atlantic, the shores of which converge toward the North 

 Pole, the tide rolling up from the southern sea is constrained 

 to rise to several times the height it had in the more open 

 water. So, too, wdien a bay is more broad-mouthed and 

 tapers to a sharp head, as is the case in the Bay of Fundy or 

 the mouth of the Severn, the tidal wave is yet further con- 

 strained and forced up, it may be, to an elevation of fifty feet 

 or more above tlie lowest level of the sea. Every considerable 

 variation in the form of the shore has its effect upon the rise 



