32 THE BIOLOGY OF THE SEA-SHORE 



doubt depend to some extent upon the volume and period 

 of such southern or northern invasions ; but they may also 

 depend upon other factors, such as the weather (temperature, 

 sunshine, rainfall, wind, etc.), at the time and previously. 



Wave Action. — This has a very decided effect upon shore 

 life and varies according to the depth of water, exposure to 

 prevailing winds and their relative force, shelter (visible or 

 submerged), character of shore-line, and other factors. The 

 following remarks are taken largely from Johnson's recent 

 work, already referred to. 



Waves are of two chief kinds : waves of oscillation and 

 waves of translation. In waves of oscillation in deep water, 

 each water particle moves through a circular orbit, the 

 particle moving forward on the crest of the wave, downward 

 on the back, backward in the trough, and upward on the 

 wave front. When this form of wave passes into shallow 

 water, it becomes higher and shorter, the front steepens, 

 the crest arches forward, and, finding itself unsupported 

 by sufficient water on the front of the wave, dashes down- 

 ward in a roar, producing the phenomenon known as the 

 " surf " ; the individual breaking wave being known as a 

 " breaker," or less frequently as a combing wave. 



The initial height of waves of this type depends upon 

 (i) the strength of the wind, (2) its duration, (3) the extent 

 of open water over which it blows. A faint breeze sets in 

 motion very small waves, which increase in size to a certain 

 limit, but which would never become billows. In the trade 

 wind belt the maximum height of a wave for a certain strength 

 of wind is soon reached, and, although the wind may con- 

 tinue steadily for days at a given strength, there is no 

 increase in the size of the waves. In very severe storms the 

 highest waves may not occur when the wind velocity is at 

 a maximum, but may develop as the wind begins to subside, 

 the excessive force having blown off the wave-tops and cast 

 them into the preceding troughs. The size of waves may 

 increase very rapidly, and Cornish records an increase of 

 seven feet during a squall lasting four minutes. The follow- 

 ing are approximate wave heights : Mediterranean Sea, 



