46 



The life of a wave may be divided into three stages. During 

 the first, it is generated by the action of wind at some distant 

 place, and in the case of a swell it forms one of a regular series 

 which travel from the deep water towards the shore. In its 

 second stage it has arrived in shoaling water and the sea bottom 

 begins to have an influence on its shape and speed. The effect 

 of the land is to retard the wave and alter the wave length (the 

 wave length is the distance from crest to crest between two 

 consecutive waA^es of the same series). As the velocity 

 diminishes, the wave length shortens in such a manner that the 

 j)eriod remains unchanged. The period is the number of 

 seconds taken by the waA^e to travel its own length. 



It follows therefore that the time between the passage of two 

 consecutive waves of a series across any fixed i)oint of observation 

 is the same wherever the point may be chosen, in deep water or 

 near the beach. This is a most useful property as it enables us 

 to learn much about the behaviour of the waves at some distance 

 from the land by merely observing the number of waves which 

 reach the shore in a given time. It can be shewn that the Avave 

 length in feet is equal to the square of the period in seconds 

 multiplied by 5|- for a wave in deep water, hence the importance 

 of being able to measure the jDcriod conveniently. The change 

 in wave length experienced in the second stage leads to an 

 increase in the height of the wave and finally when the increase 

 in height has caused the angle at the crest as seen in profile, 

 to become less than 120 degrees the third stage is reached 

 when the wave becomes a breaker and the water at the crest 

 is thrown violently forward. 



The particles of water in the surface of a wave are found to 

 be moving in vertical circles whose diameter is determined by 

 the height of the wave, and as each particle is in a slightly more 

 or less advanced stage of its revolution than its neighbours in 

 the direction of motion of the wave we get the characteristic 

 trochoidal outline of a wave seen in profile. The particles just 

 below the surface are revolving in circles of slightly less 

 diameter and so on as we go downwards. The ratio in which 

 the motion decreases depends on the wave length of the wave. 

 The diameter of the circle of motion becomes halved for a depth 

 equal to one ninth of the wave length and at a depth equal to 

 the wave length, the diameter is only one five hundredth of that 

 at the surface. 



