J. EB. Hilgard—Tidal Waves and Currents. 119 
An examination of this 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 Newcastle, a subsequent loss from 
friction in a narrow channel of nearly uniform character, and 
correspondingly a rapid propagation of the tide-wave through 
along the narrow channel of the river. At the mouth of the 
bay the duration of the flood tide is equal to that of the ebb, 
while at Philadelphia it is less by two hours forty-two minutes. 
When the tide is very large compared with the depth of water, 
this inequality becomes very great; thus, in the Severn river 
at Newnham, above Bristol, England, the whole rise of eighteen 
feet — place in one and a half hours, while the fall occupies 
ten hours. 
2. Trat CuRRENTS. 
The agency of tidal currents in producing changes in the 
entrances of bays and harbors, is a subject of the first import- 
ance to commerce and navigation, and has received full atten- 
tion in the prosecution of the American Coast Sarvey. The 
laws according to which the changes takes place require to be 
studied by long-continued observation, and when the change 
is for the worse, the means of counteracting it must be pointed 
out. 
reached during the ebb between the basin and the ocean, which 
determines the greatest velocity and transporting power reached 
by the ebb stream. 
On the bars of most of the sand-barred harbors on our south- 
ern coast, the place and direction of the channel are frequently 
