138 



WIND CURRENTS AND WIND WAVES 



mately the greatest observed heights of waves of stated periods up to 

 14 sec, and the last three lines of the table correspond to big swells. 

 The last four columns of the table give the velocities of the water at the 

 surface and at the depths of 2, 20, and 100 m. It is seen that the surface 



Table 16 



VELOCITIES OF WATER PARTICLES AT DIFFERENT DEPTHS IN SUR- 

 FACE WAVES OF DIFFERENT PERIODS, LENGTHS, AND HEIGHTS 



velocities can reach very appreciable values, up to 250 cm/sec or more, but 

 in the case of the shorter waves the velocity decreases very rapidly with 

 depth and is negligible shortly below 20 m. In waves of periods below 

 10 sec the wave motion is negligible below 100 m. The tabulated 

 velocities correspond to the heights that are entered in the table, and at 

 different wave heights the velocity will be altered proportionately. 



No measurements are available of the actual motion of water particles 

 in waves. Experience in submarines has shown, however, that the wave 

 motion decreases rapidly with increasing depth. Vening Meinesz has 

 availed himself of this fact and has been able to conduct observations of 

 gravity at sea on board a submarine, making use of pendulums, which 

 can be employed only when the motion of the vessel is small. 



A consequence of the decrease of the particle velocity with depth is 

 that a small transport of water will take place in the direction of progress. 

 A water particle will move in the direction of progress when it is above 

 its mean depth, and in the opposite direction when it is below its mean 

 depth; owing to the decrease of velocity with depth, it will move some- 

 what faster in the direction of progress than in the opposite direction. 

 Consequently, after completing one revolution in its orbit, the particle 

 will not return to the point from which it started, but it has advanced 



