TIDES, WA^'ES 105 



to some extent before the wind. On advanc- 

 ing from the open sea into water so shallow 

 that the wave motion is sensible down 

 to the bottom, the wave is retarded at the 

 bottom, its velocity and length are diminished, 

 its height increased, and the top of the wave 

 pitches forward as surf. Thus in strong winds 

 and in shallow water there is a forward move- 

 ment of some of the water in a wave, which is 

 then called a wave of translation. On strikincr 

 a shore obliquely waves may gi%'e rise to a 

 shore current. Violent storms give rise to 

 waves which run far from their place of origin, 

 diminishing in height but retaining their 

 length and velocity if unobstructed, and con- 

 stituting a swell or " ground-swell." The 

 sea-surface in the open ocean is probably 

 seldom quite smooth, there being generally a 

 " ground-swell " due to storms in different 

 places, sometimes at great distances. 



The height and length of waves, and the 

 depth to which their effect is felt, depend 

 on the depth and extent or " fetch " of the 

 ocean in which they occur. The greatest 

 waves are believed to occur in the North Atlan- 

 tic and in the great Southern Ocean, where 

 waves 560 feet in length and 50 to 60 feet in 

 height are recorded. Off the north coasts of 

 Scotland fine particles of sand are believed 

 to be moved about on the bottom at a depth 



