DIRECTION OF SWELL 67 



the latter. But the transformation is often very rapid when the wind 

 dies down, as every mariner knows. On a recent occasion, we. noted 

 that a low, but rough, sea about 2 to 3 feet high had become trans- 

 formed into a swell, though still nearly as complex in pattern, during 

 a period of about 2 hours, as the breeze slackened ; the smaller wavelets 

 were almost entirely absorbed into the higher and longer ones during 

 the ensuing half hour, by which time the wind had entirely died out. 

 And the alteration in character from sea to swell may be as rapid for 

 larger waves as for small ones if the wind falls flat, though the in- 

 corporation of the younger waves into the older and longer ones on 

 which they are running requires a longer time when the parent seas 

 are large than when they are small. 



Very few observations have been made as to how rapidly the height 

 of a swell decreases in calm weather. In one published instance, the 

 heights of swells running from the West Wind Belt in the southern 

 Indian Ocean were described as decreasing by about one-half in a 

 distance of 350 miles. And we have ourselves seen a small swell of 

 about 2 feet fall to about 3 inches in a little less than one hour during 

 a flat calm. 



THE DIRECTIONS OF SWELLS 



The directions in which swells advance are reminiscent not of the 

 winds at the time of observation, but either of winds that blew pre- 

 viously or of wind systems at a distance. Consequently, the swell that 

 is encountered at sea on any given occasion may be coming with the 

 wind; it may run against the wind (if the latter is not strong enough 

 to have flattened the waves down) ; it may run at any angle with the 

 wind ; or it may run ahead of the wind so that a ship in the path of 

 an oncoming storm may find herself plunging into swells so heavy 

 that she takes water over the bow, even if the weather is perfectly 

 calm for the time being. But the direction of advance of a swell, if 

 reversed, leads back toward the place where its parent waves were 

 produced, because waves once set in motion continue to progress in 

 their original direction for as long as they continue in deep water, 

 regardless of any subsequent changes in the direction of the wind 

 (fig. 20). If the swell continues to come from the same direction, 

 it may be assumed that the storm area as a whole is either advancing 

 directly toward the observer, that it is receding directly away from 

 him, or that it is stationary for the time being. If, however, the swells 

 are coming from a storm that is passing by, their direction of advance 

 will change as illustrated in figure 20. 



Thus the sudden development of a heavy swell at sea, or of a surf 

 upon the coast, may give warning in this way of the approach of a 



226794 O - 53 - 6 



