1S89.] l0«^ [Haupt. 



currents are due to surf<ice oscillations, ■which are interrupted and detlected 

 by the form of the shore line, as along the coast. 



With reference to the effects of prevailing winds in moving material, 

 Prof. Henry Mitchell, of the U. S. Coast and Geodetic Survey, says : 



" The motion of the waves is not always in the direction of the prevail- 

 ing winds. This fact is noted in many publications. An example of this 

 is shown by the action of the waves on the north side of Long Island, N. 

 Y., which drifts the material westward, while on the south side the mo- 

 tion of the drifted material is eastward,* and yet the prevailing winds must 

 be essentially the same on the two sides of the island. Another example 

 is furnished by Lake Michigan. On the west side, south of Milwaukee, 

 the prevailing motion is southward, and north of that place it is north- 

 ward, and yet the prevailing wind must be the same. The prevailing 

 wave motion must be influenced by the tendency which wave oscillations 

 have to move from the deep waters as a centre towards the shores. In 

 some instances the prevailing drift, too, must be modified by the pre- 

 vailing action of the littoral currents." 



In short, the oscillations of the flood tide in deep water become con- 

 verted into waves of translation on shelving sliores, where they break at 

 a permanent angle, and also generate littoral currents, both of which com- 

 bine to move the beach material in the direction of the receding coast line- 



FLOOD vs. EBB CURRENTS. 



Again, I believe it to be an error to attribute the deep holes in the 

 gorges of inlets to ebb action chiefly. In Ex. Doc. 78, Forty eighth Con- 

 gress, in reference to the Narrows of New York bay, it is said: "The 

 mean ordinary velocity at the Narrows is, during the ebb tide, about two 

 feet per second, and from this a depth of 100 feet results." In view of 

 this statement, it is strange that a greater mean ebb velocity over Five- 

 mile Bar in the Delaware is able to maintain only about seven feet of 

 depth. In fact, it is not so much a question of velocity as of reaction, 

 resulting from the compression of the flood in its efforts to pass through 

 the gorge. The surveys show that the bottom currents run flood for about 

 eleven hours out of twelve, and that the resultant of all the currents, ebb 

 and flood, is strongly up stream. It is a notorious fact that refuse, etc., 

 dumped in the lower bay, is carried by the flood to the upper bay, and it 

 certainly will not be claimed that this effect is produced by prevalent 

 storm-winds or waves. The flood resultant is also lower than that of the 

 ebb, because of its greater density. Moreover, there can be no doubt 

 that the extension of Cape Henlopen to the northward about 800 feet and 

 the deposit there of over 5,000,000 cubic yards in the last half century, in 

 opposition to the strong ebb, aided by the breakwater, and of tlie action 

 of the N.E. and N.W. storms, and the cutting away of the outer beach 

 about 600 feet near the point, is additional evidence that the flood compo- 



* This is only true for tlie eastern end of Long Island. — L. M. H. 



PROC. AMER. PHILOS. SOC. XXVI. 129. T. PRINTED MARCH 37, 1889. 



