220 ON TIDES AND TIDAL ACTION IN HARBORS. 



An examination of tbis table will show, besides the marked increase 

 in the height of the tide due to the contraction of the shores from the 

 capes up to 'New Castle, a subsequent loss from friction in a narrow 

 channel of nearly uniform character, and correspondingly a rapid propa- 

 gation of the .tide-wave through the deep water of the bay, and a com- 

 paratively slow movement along the narrower channel of the river. At 

 the mouth of the bay the duration of rise exceeds that of fall by ten 

 minutes, while at Philadelphia it is less by two hours forty-two min- 

 utes. When the tide is very large compared with the depth of water, 

 this inequality becomes very great; thus, in the Severn River, atNewn- 

 ham, above Bristol, England, the whole rise of eighteen feet takes place 

 in one and a half hours, while the fall occupies ten hours. 



TIDAL CURRENTS. 



The agency of tidal currents in produciug changes in the entrauces 

 of bays and harbors is a subject of the first importance to commerce 

 and navigation, and has received full attention in the prosecution of the 

 American coast survey. The laws according to which the changes 

 take place require to be studied by long-continued observation, and 

 when the change is for the worse, the means of counteractiug it must 

 be pointed out. 



As on the average the same amount of water moves inward and out- 

 ward with the flood and ebb tides, we might readily suppose that the 

 same amount of material is transported either w^ay, and that no impor- 

 tant change would take place in the configuration of the bottom. But 

 the operation of the flood-stream is very different from that of the ebb- 

 stream. We have, as a general feature, an interior basin of some ex- 

 tent, communicating with the sea by a comparatively narrow i^assage. 

 The flood-stream, therefore, running with considerable velocity through 

 this channel, will, as it enters the basin, spread out and become slow, 

 depositing the sand and mud it is charged with, and making extensive 

 flats or shoals opposite the entrance. The ebb-stream runs slowly over 

 the flats from all directions toward the opening without removing much 

 of the deposit, and gradually concentrates in definite narrow channels, 

 which it scoops out, and the depth of which will depend in a great de- 

 gree on the proportion of the area of the basin to the outlet, or, in other 

 terms, on the difference of level which will be reached during the ebb 

 between the basin and the ocean, which determines the greatest veloc- 

 ity and transporting power reached by the ebb-stream. 



On the bars of most of the sand-barred harbors on our southern coast, 

 the place and direction of the channel are frequently changed during 

 violent storms ; when the direction of the waves happens to be oblique to 

 that of the channel, or when the sea runs directly upon the channel, the 

 depth of water may be considerably diminished for the time being by the 

 sand rolled up by the waves. But in all these cases it is found that 

 the normal depth is speedily restored by the scour of the ebb-tide, which 



