47 



These figures refer to waves in deep water and when the wave 

 reaches shoaling water the numbers have to be modified. The 

 motion of the particles is also distorted and instead of circles 

 we find the particles near the bottom moving in ellipses. The 

 action of a wave on the water at the bottom is now such that the 

 particles after rising slightly are carried forward nearly 

 horizontally returning to their original position by a nearly 

 horizontal motion at a lower level. The ratio of the horizontal 

 motion at the bottom to that at the surface depends on the depth 

 of water relative to the wave length and this relation is shown 

 in the Table. 



It will be clear that it is a matter of great importance to be 

 able to determine the wave length at various times of the year, if 

 we are to learn anything of the conditions under which the crab 

 pots are exposed. During the past year I have been able to get 

 an automatic record of the period of the North Sea waves by 

 means of an apparatus placed in the lighthouse of the South 

 Shields pier by the kind permission of the Tyne Improvement 

 Commissioners. It has been possible also to obtain a rough 

 estimate of the height of the waves. These records show that 

 the periods vary from 3 seconds to 12 seconds although the latter 

 value occurs but rarely. During the winter months the average 

 23eriod is 5 to C seconds, which corresponds to a wave length of 

 about 156 feet in the open sea. The average depth of crab pots 

 would be about 21 fathoms, and on reaching this depth the 

 waves would have a length of roughly 150 feet, and a velocity of 

 about 30 feet per second. The maximum velocity of the water 

 particles at the surface is therefore also 30 feet per second, which 

 gives for the horizontal velocity at the bottom only 4 inches per 

 second. But with a long period wave having a wave length of 

 say, 500 feet, the A^elocity at the bottom would be one half of that 

 at the surface and there is little difficulty in accounting for the 

 damage done to crab pots by waves if they are accompanied by 

 motions of the water on this scale. 



The elliptical motion of the water plays an important part in 

 the transport of shingle and sand. A body which settles down 

 moderately slowly in water will have a good chance of being 

 transported rapidly. A body which allowed itself to be carried 

 forward bv the water, but anchored itself when the back flow 



