WIND CURRENTS AND WIND WAVES 1 35 



sizes are present, varjdng in form from long, gently sloping ridges to 

 waves of short and sharp crests. Superimposed on the gentler waves, 

 which may or may not run in the direction of the wind, appear series of 

 deformations of the surface which, from the point of view of physics, can 

 be termed 'Svaves'' only by stretching the definition. 



In spite of the irregular appearance of the sea, it is possible to apply 

 the terms wave 'period, T, wave height, H, and wave length, L, because some 

 of the waves will be more conspicuous than others and their character- 

 istics can be observed. The general theory of waves on the surface of 

 the sea leads to a simple formula for wave velocity: 



L _ FL 



2. = ^2^' (VII, 25) 



from which the relations 



L = — c2 = #- 7^2 (YII 26) 



9 27r 



and T = ^^-^ = ?^c (VII, 27) 



are derived. These formulas applj^ only to a wave whose amplitude is 

 small relative to the length, and therefore they cannot be expected to be 

 valid in all cases. 



Of the three interrelated quantities, c, T, and L, the wave period T 

 can probably be most easily determined at sea by using the method 

 proposed by Cornish, which consists in recording the time intervals 

 between appearances of a well-defined patch of foam at a sufficient 

 distance from the ship. The same method can be used on the coast, 

 where, in addition, the interval between breakers can be accurately 

 timed. At sea the wave length is mostly estimated on the basis of the 

 ship's length, but this procedure leads to uncertain results because it is 

 often difficult to locate both crests of the wave relative to the ship, and 

 also because of disturbance due to the waves created by the moving 

 ship. The most satisfactory measurements are made from a ship that is 

 hove to. Another method consists in letting out a floater as a wave 

 crest passes the stern of a ship and recording the length of line paid out 

 when the floater reappears on the crest aft of the ship, as well as the angle 

 that the line forms with the direction in which the ship travels. The 

 velocity of the wave can be found by recording the time in which the wave 

 runs a measured distance along the ship. If the period is also determined, 

 the wave length is found from the simple formula L = cT. 



A large number of measurements have been made at sea in order to 

 establish the relationship between the period of wave, the wave length, 

 and the wave velocity. Critical examination of the methods employed 

 has been made, especially by Cornish, and a number of average results 



