212 DYNAMICAL GEOLOGY. 



and at each ebb, on an average, 448,000,000 cubic feet pass from the Sound 

 westward. 



Again, where the tide up a large river is detained at the head of a gradu- 

 ally contracting estuary by sand-bars with only narrow passages, it some- 

 times moves up the river all at once in one or a few great waves, producing 

 ■Avhat is called an eager (or bore) ; as at the mouth of the Hoogly (one of the 

 mouths of the Ganges), on the Tsien-Tang in China, and on the Amazon. In 

 the Tsien-Tang, at the equinoxes, the wave moves as a foaming wall of water, 

 20 feet or more high and four or five miles broad, and thus it passes Hang- 

 Chau-Fu at a rate of 25 miles an hour, dying out about 80 miles above. The 

 change from ebb to flood tide is almost instantaneous. (Macgowan, 1855.) 



At the large northern mouth of the Amazon, the pororoca, as it is there 

 called, passes up the stream in three or four closely following waves, each 

 15 to 20 feet high. As soon as the previous tide stops running out, the 

 approaching wave is seen as a white line on the eastern horizon ; onward it 

 comes with rumbling sounds (imitated in the word pororoca) that grow 

 louder and louder ; finally it rushes forward over the top of the long wall, 

 like an endless cataract, in quick pursuit by the other waves ; and continues 

 up the river for 70 or 80 miles, or two thirds of the way to Macapa. (J. C. 

 Branner, 1884.) 



Rivers with open mouths receive the tidal wave quietly and carry it as far 

 within as high-tide level goes, the movement being communicated to the 

 water of the river, and the salt water following for part of the distance, and 

 ending as an under-current. It extends up the Amazon to Obidos, nearly 

 500 miles ; up the Hudson to Troy, 150 miles, two waves being in the river 

 at once ; up the Connecticut to Hartford, 50 miles. Rising above the level 

 of the wells along the coast and the outlets of subterranean streams, it raises 

 their waters, so that such wells also have their tides. 



In seas more or less shut in from the ocean and outside of the general 

 course of the tidal wave, the tides are small. In the Mediterranean, for 

 example, the tide is perceived only at the ends of bays, as at Venice in the 

 Adriatic. 



In consequence of the tidal movement the sea has its flood-grounds, like 

 rivers ; but the floods occur twice a day, with each recurring tide. 



At some places in the Pacific, owing to tlie conjunction of tidal waves, high water 

 occurs uniformly at 12 h., and low at 6 h. This is the case at Tahiti, where the tide has a 

 height of 1 to l^- feet. The author governed himself accordingly in his excursions at low 

 water over the coral reefs. 



2. Winds : Wind-made Waves and Currents. 



As the great currents of the oceans — the Atlantic and others — are 

 attributed by many to the action of the regular winds, these currents may 

 here come under consideration as well as those made by storm-winds. But 

 the currents made by the storm-winds, that is, the littoral currents and the 



