ON TIDES AND TIDAL ACTION IN HARBORS. 



219 



Now, where a bay or indentation of the coast presents itself, oi^eniug 

 favorably to the tide- wave thus developed, and decreases in width from 

 its entrance toward its head, the tide rises higher and higher from the 

 mouth upward. This is due to the concentration of the wave by the 

 approach of the shores and to the gradual shoaling of the.bottom. 



This effect is strikingly ilhistrated by a generalization of the heights 

 of the tides on the Atlantic coast of the United States. That coast 

 presents, in its general outline, as represented in the annexed diagram, 

 three large bays : the great southern, from Cape Florida to Cape Ilat- 

 teras ; the great middle, from Cape Hatteras to Nantucket ; and the 

 great eastern, from Nantucket to Cape Sable, now known as the Gulf 

 of Maine. It will be seen that the tide-wave arrives at about the same 

 time at the headlands, Cape Florida, Cape Hatteras, Nantucket, and 

 Cape Sable, and that at those points the height is inconsiderable com- 

 pared with the rise at the head of the several bays. Thus, at Cape 

 Florida the mean rise and fall is only one and one-half of a foot ; at 

 Hatteras, but two feet ; while at the intermediate entrance to Savannah 

 it reaches seven feet, declining in height toward both capes. Again, at 

 the head of the middle bay, in New York Harbor, it reaches five feet, 

 while on the southeast side of Nantucket Island it is little over one 

 foot. The configuration of the eastern bay is less regular, and the cor- 

 respondence of heights is not so obvious. The recess of Massachusetts 

 Bay is well marked, the increase in height reaching ten feet at Boston 

 and Plymouth. Boiling on eastward along the coast of Maine, it con- 

 stantly increases; but the most striking effect of the convergence of 

 shores is exhibited in the Bay of Fundy. At St. John's the mean height 

 of tide is nineteen feet, and at Sackville, in Cumberland Basin, thirty- 

 six feet, attaining to fifty feet and more at spring-tides. 



When the wave leaves the open sea, its front slope and rear slope are 

 equal in length and similar in form, but as it advances into a narrow 

 channel, bay, or river, its front slope becomes short and steep, and its 

 rear slope becomes long and less inclined. Hence arises the fact that 

 at a station near the sea, the time occupied by the rise is equal to that 

 occupied by the descent ; but at a station more removed from the sea, 

 the rise occupies a shorter time than the descent. Thus, in Delaware 

 Baj' and Eiver we have the following relations of the duration and 

 height of rise and fall : 



