THE OCEAN. 671 



fng bottom, change from oscillatory to translation waves, that is, to 

 those in which there is actual forward movement in the water. They 

 " break " when the depth of the water is a little more than the height 

 of the crest above the undisturbed level or plane of rest. At the 

 instant of breaking, the upper part is thrown forward, while the 

 lower part flows backward as the " under-tow." 



If there is deep water quite up to vertical shore cliffs, the waves 

 rise and fall but do not break, and the action has small mechanical 

 force. But on shelving shores, up which the waves advance and 

 plunge, the force is often very great. 



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, 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. During a westerly gale in March, 1845, his dynamom- 

 eter registered a pressure of 6,083 lbs. per square foot, which gives 



for the velocity per second, by the formula, .1 Q ( P being the press- 

 ure in pounds and 64 the weight of a cubic foot of sea-water), 

 '" = 53*32 feet. The hydrostatic pressure due to a wave 



j 



64 



20 feet high is only about half a ton to the square foot, and the rest 

 of the force comes from its velocity. Mr. Stevenson states that on 

 one of the Hebrides a mass of rock of about 42 tons weight was 

 gradually moved in a storm five feet ; with each incoming wave it was 

 made to lean landward, and the back run uplifted it with a jerk, leav- 

 ing it with little water about it. Hagen reports that in the harbor of 

 Cette a block of concrete, measuring 2,500 cubic feet and weighing 

 probably 125 tons, was moved on its bed over three feet 



2. Currents. — Winds also cause currents. The prevailing winds of 

 an ocean, like the trades (p. 43), cause a parallel movement in the 

 surface-waters ; and, when the direction is reversed for half the year, 

 as in the western half of the tropical Pacific, the current is changed 

 accordingly. These currents become marked along shores, and es- 

 pecially through open channels. The great currents of the ocean are 

 attributed by some physicists to the force of the prevailing winds. 

 Prolonged storms often produce their own currents, even in mid-ocean, 

 and more strikingly still among the bays and inlets of a coast. 



The forcing of waters into bays, whether by regular winds or by 

 storms, causes a strong under-current outward, like that from the 



