654 DYNAMICAL GEOLOGY. 



current, which is directly the counterpart of the in-flowing current. 

 It is more quiet than the latter in its movement, although often 

 a rapid and powerful current, because more contracted in width, — 

 and especially so in bays, where the waters of a river add to the 

 volume of the ebb. Wind-waves may increase greatly the force of 

 the in-coming tide, but not so with the out-flowing, since waves 

 always act shoreward. 



The piling of the tidal waters to an unusual height in converging 

 bays, raising them far above their level outside, is another powerful 

 cause of out-flowing currents. The flow is along the bottom ; and 

 in a case like that of the Bay of Fundy it must have great power. 



3. Ordinary wind-waves and currents. 



1. Waves. — The winds are almost an incessant wave-making 

 power. Even in the calmest weather there is some breaking of 

 wavelets against the rocky headlands or the exposed beach ; and 

 with ordinary breezes the beaches and rocks are ever under the 

 beating surge, night and day, from year to year. Most seas, more- 

 over, have their storms, and in some, as those about Cape Horn, 

 gales prevail at all seasons. The breakers on the shores of the 

 Pacific are especially heavy, on account of its extent and depth. 



Through a large part of the ocean the winds are constant in 

 direction, either for the year or half-year. 



Stevenson, in his experiments at Skerry vore (west of Scotland), 

 found the average force of the waves for the five summer months 

 to be 611 pounds per square foot, and for the six winter months, 

 2086 pounds. He mentions that the Bell Rock Lighthouse, 112 

 feet high, is sometimes buried in spray from ground-swells when 

 there is no wind, and that on November 20, 1827, the spray was 

 thrown to a height of 117 feet, — equivalent to a pressure of nearly 

 three tons per square foot. 



2. Surface-currents. — Winds also cause currents. The prevailing 

 winds of an ocean, like the trades (p. 44), cause a parallel move- 

 ment in the surface-waters ; and when the direction is reversed for 

 half the year, as in the western half of the tropical Pacific, the cur- 

 rent is changed accordingly. These currents become marked along 

 shores, and especially through open channels. Prolonged storms 

 often produce their own currents, even in mid-ocean, and more 

 strikingly still among the bays and inlets of a coast. 



These currents made by the winds are inferior in power to the 

 tidal currents among the inlets and islands of a continental coast ; 

 but about oceanic islands they are often of greater strength. 



