2 24 SEA AND LAND 



\ariation in the amount of tides in different parts of the ocean 

 and even on neighborin^j^ portions of the same shore the work 

 which they perform is exceedingly variable as regards the 

 measure of the effects, but it is almost everywhere of great 

 importance, and therefore merits careful consideration. 



At first sight it might seem that, inasmuch as the tide 

 enters and leaves a harbor with the same volume and velocity 

 in the movements of flow and ebb, the effect of these con- 

 trasted motions would neutralize each other. This, however, 

 as w^e shall see, is not exactly the case. There is always a 

 difference in the effect of the incoming and the outgoing 

 tide which though not great is of momentous importance 

 from its accumulated effects. The nature of this difference 

 is easil)' apprehended. When the tide enters an inlet it gen- 

 erally flows up an inclined bottom extending from the outer 

 sea to the head of the recess and when it ebbs away it moves 

 down this slope or rather, we should say, out over it. In the 

 movement of the flood the tidal current if it have any con- 

 siderable energy bears a certain amount of detritus up the 

 incline toward the interior of the reentrant. The distance 

 to which it will be carried will depend upon the energy of 

 this current and the rise of the slope over which it moves. 

 In the reflux the tide will convey the same detritus with the 

 same energy of current, but as it is carrying the debris down 

 a slope it can carry it farther outward than it did inward. 



The ordinary action of the tide, operating in the manner 

 just dc?scribed, is of itself sufficient to cause its currents to 

 scour away materials from the open shores as well as from 

 harbors and to convey them out to sea to a depth of water 

 where the currents are no longer sufficient to bear them 

 farther. In many if not in most bays and harbors there are 



