IS'^O.] 155 [Haupt. 



roll up the sand diagonally. The particles may possibly return normally 

 with tlie under-tow, only to be again transported obliquely, and by this 

 zigzag path it will advance in the direction of the receding beach ; a lit- 

 toral current merely intensifies this action. 



This movement along shore is, therefore, largely dependent on the 

 angle at which the flood breaks upon the shore, and this angle is practi- 

 cal!}' a constant for a particular place, modified by the wind. But vari- 

 ability in the wind is not the controlling condition. It may sometimes 

 increase the littoral drift, and at others neutralize it entirely. While there 

 may be a prevailing north-east wind, as alleged, it would seem from an 

 examination of the hydrographic charts, that the prevailing winds are off 

 shore and the greatest storms from the south and west. In the middle 

 bay particularly, extending from Cape Hatteras to Nantucket, the on- 

 shore winds are limited to a few months during the summer. It would 

 appear from these charts that the prevailing winds, and consequently the 

 wind waves, can have very little influence in transporting material along 

 the shores at or below the water line. 



With reference to the existence of a constant angle for the breaking 

 wave as well as of a littoral current. Prof. Henry Mitchell, of the U. S. 

 Coast and Geodetic Survey, says : "From considerable experience in the 

 study of waves upon the open coast, I have come to the conclusion that 

 there is everywhere a prevalent, if not a permanent, angle at which the 

 larger class of swell or rollers strike the general shore line ;" also, "the 

 coast currents in some places have a velocity of one-third of a mile per 

 hour in thirty fathoms of water. They are in some localities nearly par- 

 allel, in others normal to the general trend of the shore line, and, so far 

 as the few observations we have seen may indicate, the directions of ebb 

 and flood are not usually opposed, although lying at an oblique angle with 

 each other." 



Dr. Whewell says, concerning the action of the flood tide : " The cotidal 

 lines make a very acute angle with the shore line, and run for great dis- 

 tances nearly parallel to it. They are convex in the direction of their 

 motion, the ends near the shore being held back by the smaller velocities 

 in shallower water and other resistances." 



But there can be no holding back without a reaction upon the shores, 

 whereby the sandy particles would be dragged by the friction in the direc- 

 tion of this movement. 



Mr. E. A. Geiseler, C.E., formerly Assistant U. S. Engineer and Super- 

 intendent of Construction on Lighthouse Service, says : "I fully coincide 

 with Prof. Haupt in his opinions that littoral currents are produced by 

 the entrance of the tidal wave into bays. From the higher crest the 

 water must flow at first vertically to such crest towards the shore line, and 

 on approaching the latter be gradually deflected into a direction parallel 

 to it." 



From the reference of the "tidal currents," to me personally, as their 

 discoverer or imagiuer (see quotation), it is necessary here incidentally to 



