St 



KNOWLEDGE. 



[March 2, 1890. 



pendulous character of the wave), we may calculate 

 tbe lengths of wave which correspond to the above periods 

 of oscillation. A wave-length of fifty feet corresponds to 

 a period of two and a quarter seconds, and of three 

 hundred and ten feet to five and a half seconds. The 

 greater the natural time of swing of a ship, the loss 

 likelihood is there of encountering waves whose period 

 coincides with that of the ship ; for it often happens that 

 the longer waves are absent, whereas in a storm the wind 

 is constantly forming new waves which grow in sine, so 

 that there are present at the same time waves of all sizes, 

 and, therefore, of all periods up to the largest. A small 

 ship is sure to meet with undulations coinciding with its 

 time of swing, whereas a large ship may often escape. 



As tlie billows travel onwards the energy is passed on 

 from point to point, silently, smoothly, till the leeward 

 shore is reached. Here all is changed. On the one side is the 

 swinging water, ever handing on the energy of its motion. 

 On the other side is the dead resistance of the beach, to 



down to the base of the cliff by these agencies is removed 

 by the sea, so that the cliff is maintained at an angle 

 steeper than the angle of repose, and is constantly falling. 

 When the jointing planes of a rocky cliff slope downwards 

 from the shore line the waves undermine the cliff, and great 

 masses of superincumbent rock fall by their own weight, 

 as the lumps of coal fall in a mine through the skilful 

 undercutting of the collier. 



The work of tlie waves is assisted not only by the flux 

 and reflux of the tides, but also by the " heave of the sea " ; 

 that is to say, the bodily conveyance of water by wind. 

 When wind acts upon the surface of the sea it not only 

 carves the surface into waves, but by the exercise of a power- 

 ful tangential force, heaps up the water upon the leeward 

 shore. Something of the kind may be observed in a roadside 

 puddle on a squally day. When a gust of wind has passed 

 the water springs back suddenly to windward. In wind- 

 forced waves the motion is not precisely a completed circle 

 or ellipse. At each swing the water particle is brought 



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which each breaker as it falls yields up its store of energy. 

 There is no finer display of natural forces than the rush of 

 the waves on a rock-bound coast, when each billow as it 

 nears the shore raises a steeper crest, and, dashing down 

 in thunder on the shore, throws upwards and abroad a 

 cloud of glittering spray which falls in salt showers upon 

 the tremblii'g rocks. 



Everyone is familiar with the work of the breakers in 

 tearing down cliffs and grinding the fragments into shingle 

 and sand ; but it may easily escape notice that the forma- 

 tion of chft's is also the work of the sea. The space through 

 which the breakers act is chiefly that between high and low 

 water mark, between which a sloping shore is cut away so as 

 to form a nearly flat beach terminated by a cliff. In point 

 of fact the destruction and the formation of cliff's is the 

 same process. Sometimes the waves pile up a bank of sand 

 or shingle which protects the chft" from the direct action of 

 the breakers. The cliff', however, gives way under the 

 actions of wind, rain, and frost ; and the material carried 



slightly further forward than at the end of the previous 

 swing. The result is that a stone 0)i the bottom tends, 

 on the whole, to be pushed towards the shore, and this 

 tendency increases near the line of breakers. On the other 

 hand, the slope of the shore assists the action of the back- 

 ward swing of the water, so that at a certain depth the two 

 motions balance one another. Outside the position corre- 

 sponding to this depth a sloping sea-bottom tends to move 

 seawards. Within this line the bottom tends to move 

 shorewards, and to pile itself up into a beach or bar. If 

 the entrance of a harbour be on the landward side of this 

 line the harbour is liable to be silted up, especially in a 

 tideless sea, where there is no scour to remove the heaped-up 

 sand and shingle. 



In designing structures to resist the force of the waves 

 the marine engineer has to take account of the length from 

 crest to crest attained by the waves in the locality, for on 

 this depends the depth to which the water is agitated. 

 The time which elapses between the passage of succeeding 



