660 DYNAMICAL GEOLOGY. 



ing a bay, and especially where there is free entrance to a channel 

 from two directions. In the middle Atlantic, at St. Helena, it is two 

 or three feet ; at the Azores, three feet ; on the Atlantic coast of the 

 United States, from five to twelve feet ; but in the Persian Gulf the 

 highest tide at the extremity is thirty-six feet ; at the mouth of the 

 Severn, forty-five feet ; at the Bay of St. Michael (west coast of Nor- 

 mandy), France, forty-five to forty -eight feet ; in the Bay of Fundy, 

 forty to sixty-nine feet ; in the gulfs of San Jorge and Santa Cruz, at 

 the entrance of the Straits of Magellan, forty-eight to sixty-six feet. 

 In the central Pacific, the height is two to four feet ; and at Tahiti 

 high tide occurs always at noon. 



2. Translation Character of the Tidal Waves. — The tidal waves which 

 succeed one another around the globe become appreciably translation 

 or propelling waves, on soundings ; and directly upon a coast, espe- 

 cially along its deeper bays or inlets, they constitute a force of great 

 energy. The borders of all the continents and islands feel this power, 

 and exhibit its effects. 



3. In-flowing Tidal Currents. — The in-coming tide has a progres- 

 sive movement along a coast, varying in its effects, according to the 

 trend of the coast with reference to the course of the tidal wave. 



If a bay at the mouth of a river has a long projecting cape on the 

 side from which the wave comes, it will have usually a good depth 

 and entrance, the detritus brought clown by the outflowing tide being 

 carried out so far as to be swept off to leeward. But if the cape is 

 on the opposite side, the bay or mouth of the river will commonly be 

 choked up by sand-banks, made of the detritus thrown into it by the 

 unparried in-flowing tide. 



The tidal current becomes one of great strength, where ihere are 

 narrow channels to receive and discharge the waters. The movement 

 may have the violence of a river-torrent, when the entrance to bays 

 is of a kind to temporarily dam up the waters, until the far-advanced 

 tide has so accumulated them that they overcome the resistance and 

 pass on in a body. In the Bay of Fundy, the waters of the in- 

 coming tide are raised high above their natural elevation, so that, as 

 they advance, they seem to be pouring down a slope, making a turbid 

 waterfall of majestic extent and power, without foam. 



In some cases, the whole tide moves in all at once, in a few great 

 waves. This happens especially at the mouths of rivers, where there 

 is obstruction from sand-bars, and other favoring circumstances about 

 the entrance. The phenomenon is called an eagre or bore. The flow 

 of the tides at the Bay of Fundy has something of the character of 

 an eagre. But the most perfect examples are afforded at the mouths 

 of the rivers Amazon, Hoogly (one of the mouths of the Gauges), 



