THE OCEAN. 661 



and Tsien-tang, in China. In the case of the last-mentioned river, the 

 wave plunges on like an advancing cataract, four or five miles in 

 breadth and thirty feet high, and thus passes up the stream, to a dis- 

 tance of eighty miles, at a rate of twenty-five miles an hour. The 

 change from ebb to flood- tide is almost instantaneous. Among the 

 Chusan Islands, just south of the bay, the tidal currents run through 

 the funnel-shaped frith with a velocity of sixteen miles an hour. 

 (Macgowan.) 



In the eagre of the Amazon, the whole tide passes up the stream in 

 five or six waves, following one another in rapid succession, and each 

 twelve to fifteen feet high. 



4. Out-flowing Currents. — The ebbing tide causes an out-flowing 

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

 It is more quiet than the latter in its movement ; but it is often a 

 rapid and powerful current, because more contracted in width than 

 that of the flow, — and especially so in bays in which the waters of a 

 river add to the volume of the ebb. 



The piling of the tide-waters to an unusual height in converging 

 bays, raising them far above their level outside, is another cause of 

 out-flowing currents. The flow is along the bottom ; and it often has 

 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 waves, night 

 and day, from year to year. Most seas, moreover, 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 direc- 

 tion 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 2,086 

 pounds. He mentions that the Bell Rock Lighthouse, one hundred 

 and twelve 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 one hundred and seventeen 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. 43), cause a parallel movement 



