BY G. H. HALLIGAN. 629 



influence indirectly on the travel of beach-material, as it either 

 loosens or compacts the sand as the tide rises or falls, and thus 

 enables the prevailing wind to effect a considerable movement 

 which would be otherwise imperceptible. 



In a calm tidal estuary with a sandy shore, the water cannot, 

 of course, percolate the sand, when the tide is rising, at the same 

 rate as the water itself rises. The water in the stream is thus 

 higher than the water in the sand, and the effect is to make the 

 sand " live;" the upward pressure of the water tends to lessen 

 the cohesion of the grains. In this state it is manifest that, 

 should an alongshore current arise, the sand or shingle will be 

 easily moved. With a falling tide the water in the sand does 

 not fall at the same rate as the water in the stream, and the 

 tendency is to increase the cohesion of the grains and thus render 

 them less easily moved than during the flowing tide. The credit 

 of this interesting discovery is due to L. M. Haupt, M. Amer. 

 Soc. C.E., and was published with his paper on " Littoral Move- 

 ments on the New Jersey Coast" in 1890. Suppose now a 

 strong onshore wind arises and creates waves and a heaping up 

 of the water on the beach. In order to restore equilibrium, this 

 heaped-up water flows away seawards as an undercurrent, known 

 to surf bathers as the undertow or backwash. 



The action of the waves is to heap up the sand on the beach, 

 and of the undertow or offshore current to take the sand to the 

 sea, and the resultant movement of the beach depends upon the 

 violence of the wind. A very strong onshore wind will heap up 

 the water on the beach to a considerable extent; the violence of 

 the wind will increase the size of the waves, and thus stir up the 

 sand and shingle, and the undertow will be more effective in 

 conveying it seawards. As a result of long-continued gales, a 

 false beach or " full " is sometimes formed, consisting of the sand 

 thus transported by the undertow, and deposited where the move- 

 ment of the waves is not sufficient to hold it further in suspension. 

 When the «ales subside, this sand is gradually moved shorewai'ds 

 again, but in transit it is shifted up or clown the coast even by 

 the slowest of currents. 



