WATER AS A MECHANICAL AGENT. 215 



promotes abrasion on sea-borders in a way not possible on the shores of 

 lakes. The flow up the coast and the tidal rivers sets back the river waters, 

 gives them increased depth, and floods the tidal flats. 



Passing np large bays which gradually narrow inward, the mass of water 

 l)ecomes forced to quicker movement or greater height, or both, to keep time 

 with the advance behind ; and in such cases, coasts, against wdiich there is 

 friction, may be worn, and if shallow, some stirring up of the bottom may be 

 produced. And if, further, the waters are held back by obstructing banks 

 Tintil nearly at full tide before they move in, they may rush forward, as in 

 the eager, with greater destruction. When the eager of the Tsien-Tang is 

 approaching Hang-Chau-Fu, the boats along the shore are quickly rowed to 

 the middle of the stream and placed with the bow to the wave ; they rise 

 and fall as it passes, — about 20 feet, — and in a few minutes are back at 

 their shore traffic, — facts evincing that the waters are those of a wave, and 

 not of a current. But along shores that obstruct the movement artificial 

 embankments or dykes are often torn up. 



The eager or pororoca of the Amazon has the action of an enormous 

 plunging wave. The forest-covered land, as Branner states, is torn up to 

 ^reat depths ; forests are uprooted and swept away, the trees left matted 

 and tangled and twisted together upon the shore, or half buried in the sands, 

 "• as if they had been so many strings or bits of paper," and the region inland 

 ■over which the flood has swept is loaded with the debris. Moreover, new 

 islands of large size and new shoals and bars and channels are left behind it. 

 Branner adds that this is the work of the tidal wave, not Of a tidal current. 



2. Tlie outjloio. — By the inflow of the tidal wave a great body of water 

 along a coast is raised some feet above low-tide level, and acquires thereby 

 an amount of energy depending on the height of the tide. The energy is 

 expended during the outflow in abrasion, transportation, deposition, overcom- 

 ing friction, and in other ways ; and sometimes it is utilized for impounding 

 a portion of the water at high tide, and making it turn a water-wheel for a 

 mill or a pump. As has been remarked, it may become an important source 

 of heat to man when coal-beds are burnt out. 



It is the source of tidal currents. The ebbing waters lie on the bottom of 

 shallow bays and necessarily follow the lowest channels ; and they thus be- 

 come divided into many workers, which may severally abrade or scour the 

 bottom, though generally more or less combined in their work of transporta- 

 tion and deposition. Along the deeper middle ytortion of Long Island Sound 

 the mean velocity of the outflow is 2-8 feet per second, and of the inflow 3-2 

 feet (Haskell). 



The force of the outflowing waters through bays is augmented where 

 xivers add to the depth, and also by the additions to the waters of a bay by 

 storm-winds. 



The denuding or scouring action of the movement, added to that of the 

 inflow, is manifest not only at harbor entrances, but also over the sea-bottom 

 in its shallower parts. In Long Island Sound wherever there is any narrow- 



