io8 A TEXTBOOK OF OCEANOGRAPHY 



enables the whole internal structure of such a wave to be 

 illustrated. 



At the surface the radius of the circle described by a 

 revolving particle is equal to half the wave-height. Under- 

 surface particles describe similar circles, but with ever- 

 diminishing radii. It is beyond the scope of this work to 

 give detailed proof of the laws regulating their movements. 



Broadly speaking the heights of the subsurface trochoids 

 diminish in a geometrical progression, while the depth in- 

 creases in arithmetical progression, and the following rule is 

 approximately correct. 



The orbits and velocities of the particles of water are 

 diminished by one-half for each additional depth below the 

 mid-height of the surface wave equal to one-ninth of a wave- 

 length i.e.: 



., .. t 



Depth in fractions of a wave-length below \ , 



the mid-height of the surface wave ... / 

 Proportionate velocities and diameters ... i \ \ I /,., ; ,\, .,!,.-, etc. 



Waves of 90 metres length and 3 metres height are not 

 uncommon with strong winds in the open ocean. 



At 10 metres depth the height of the subsurface trochoid 

 would be i '5 metres; at 20 metres depth 0*75 metres; at 50 

 metres only 9 centimetres ; in 100 metres not quite 3 millimetres 

 that is, hardly perceptible. 



An ocean storm-wave 600 feet long and 40 feet high from 

 hollow to crest would have at a depth of 200 feet a subsurface 

 trochoid with a height of 5 feet ; at 400 feet ( J of the length) a 

 trochoid of 7 or 8 inches only. 



This rule and these examples are sufficient for practical 

 purposes. 



' Very often the motions of these originally vertical 

 columns of particles have been compared to those occurring 

 in a corn-field, where the stalks sway to and fro and a wave- 

 form travels across the top of the growing corn. But while 

 there are points of resemblance between the two cases, there 

 is also this important difference the corn-stalks are of constant 



