56 WAVES 



ning against the current. The proverbially choppy and irregular 

 waves of the Gulf Stream — especially when a northeast wind is blow- 

 ing against the current — have this source; so, too, the high seas that 

 develop along the easterly edge of the Grand Banks of Newfoundland 

 when southerly winds combat the Labrador Current coming from the 

 north; also off Cape Agulhas, South Africa, with westerly winds, dur- 

 ing the season of the year when the Agulhas Current is flowing west- 

 ward. And many other like cases might be cited in other parts of the 

 ocean where current and wind are opposed. 



Another phenomenon of practical importance is that when waves, 

 running against a tide rip or other contrary current, pass out of the 

 latter, they lose height and become smoother with astonishing abrupt- 

 ness, for they then shrink almost instantaneously to the heights at 

 which the winds blowing at the time would have maintained them 

 in still water, while their lengths increase at the same time. 



Likewise, storm waves may be entirely knocked down if they strike 

 a strongly running tidal current at right angles, perhaps because of 

 the eddies that are set up along the zone of conflict between the 

 current and the less rapidly moving water outside its influence. The 

 narrow waters between the Shetland Islands and around them afford 

 classic examples of this. But, for some reason, it appears that ocean 

 currents do not have a quieting effect of this sort on wave trains that 

 run transverse to them, for swells originating from storms in the 

 Gulf Stream region of the northwestern Atlantic are sometimes known 

 to run as far as St. Helena in the South Atlantic, which involves the 

 crossing of the North Equatorial Current, of the Equatorial Counter- 

 current, and of the South Equatorial Current. 



ALTERATIONS IN THE DIMENSIONS OF WAVES OVER 

 A SHOALING BOTTOM 



Waves that are generated out at sea are not interfered with by the 

 proximity of the bottom, for this happens, in measurable degree, 

 only where the depth of water is less than one-half as great as the 

 lengths of the waves (or than the lengths to which these would have 

 grown, if unhampered) ; and storm seas are seldom more than 600 to 

 800 feet long, whereas the break in slope at the margin of the Con- 

 tinental Shelf lies at a depth of about' 600 feet in most parts of the 

 world. But when waves run in from the open sea toward the coast, 

 their lengths are altered as they advance over the shoaling bottom, 

 and often to such a degree as to be of considerable importance from 

 the seaman's standpoint. This phenomenon bears so directly on the 

 development of surf that it is discussed in more detail in relation to 

 the latter ( p. 102) . It is enough here to point out that waves decreases 

 progressively in length, as the water shoals, according to the relation- 



