198 CORALS AND CORAL ISLANDS. 



their greater velocity, plunge with violence upon the barrier 

 before it. The force of the ocean's surge is therefore mostly 

 confined to the summit waters, which add weight to superior 

 velocity, and drive violently upon whatever obstacle is presented. 

 The lower waters of the surge advance steadily but more slowly, 

 owing to the retarding friction of the bottom ; the motion they 

 have is directly forward, and thus they act with little mechani- 

 cal advantage ; moreover, they gradually swell over the shores, 

 and receive, in part, the force of the upper waters. The wave, 

 after breaking, sweeps up the shore till it gradually dies away. 

 Degradation from this source is consequently most active where 

 the upper or plunging portion of the breaker strikes. 



But, further, we observe that at low-tide the sea is compara- 

 tively quiet ; it is during the influx and efflux that the surges 

 are heaviest. The action commences after the rise, is strongest 

 from half to three-fourths tide, and then diminishes again near 

 high tide. Moreover, the plunging part of the wave is raised 

 considerably above the general level of the water. From these 

 considerations, it is apparent that the line of greatest wave- 

 action must be above low-water level. Let us suppose a tide 

 of three feet, in which the action would probably be strongest 

 when the tide had risen two feet out of the three ; and let the 

 height of the advancing surge be four feet : — the wave, at the 

 time of striking, would stand, with its summit, three feet above 

 high-tide level; and from this height v/ould plunge obliquely 

 downward against the rock, or any obstacle before it. It is 

 obvious that, under such circumstances, the greatest force 

 would be felt not far from the line of high tide, or between 

 that line and three feet above it ; moreover, the rise of the 

 waters to half or two-thirds tide affords a protection against the 

 breaker to whatever is below this level. In regions where the 

 tide is higher than just supposed, as six feet for example, the 

 same height of wave would give nearly the same height to the 

 line of wave action, as compared with high-tide level. Under 

 the influence of heavier waves, such as are common during 

 storms, the line of wave-action would be at a still higher 

 elevation, as may be readily estimated by the reader. 



