DIRECTION OF ADVANCE 37 



or quite as high, for it is known to have crossed the entire breadth of 

 the Pacific Ocean in 12 hours, while a secondary wave produced by it 

 in the Atlantic Ocean, was recorded on the tide gage at Rochefort on 

 the coast of France, 2 days later (Gaillard, 1904, p. 108, and Berget, 

 1923, p. 53). The earthquake waves that did severe damage in the 

 Hawaiian Islands on April 1, 1946, offer a more recent illustration. 

 In deep water (16,800 feet) these waves theoretically were advancing 

 at about 430 knots, which agrees fairly well with the recorded interval 

 of 4 hours and 34 minutes occupied by the first of them in travelling 

 Ihe distance of 1,946 nautical miles from their point of origin south of 

 the Aleutian Islands. The waves arrived at average intervals of 15 

 to 17 minutes, indicating that their lengths in deep water had been 

 about 100 nautical miles. Waves of this sort are so long relative to 

 their heights, i. e., their slopes are so gentle, that they cannot possibly 

 be recognized by ships that meet them out at sea. 



At first thought, one might expect that the velocity with which a 

 group of waves advances as a whole would be the same as the velocities 

 of the individual waves that make up the group. And this is true of 

 waves that are still being built up by the wind. In the cases, however, 

 of old swells that continue to run on, as explained on page 34, either 

 after the wind has died down or after they have advanced beyond the 

 limits of the wind system that produced them, the leading waves tend 

 to die out, chiefly because their energy is expended in setting un- 

 disturbed water in motion, but partly because of the resistance of the 

 air that the wave crests must displace in their advance. The next 

 wave then takes the lead, and this process of replacement continues 

 progressively. Each wave then takes up energy that was left behind 

 by its predecessor, and, in turn, leaves some of its own' energy to be 

 taken up by the next wave. And new waves are formed, successively, 

 in the rear of the preexisting group, so that the position occupied by 

 the latter as a whole, as existing at any given moment, is not as far 

 advanced as it would be if it still consisted of the same individual 

 waves of which it was originally composed. Theoretically, the veloc- 

 ity of such a group of swells is only one-half as great as that of its 

 component waves, individually, if the depth of water is greater than 

 the lengths of the waves, as it actually is over the oceans as a whole. 



THE DIRECTIONS IN WHICH WIND WAVES ADVANCE 



Wind waves advance at right angles to the sidewise extension of 

 their crests, or at right angles to the chords or tangents of the latter, 

 if the crests are wide enough transversely to show a measurable cur- 

 vature. 



When the wind is rising, the waves that it generates run with it, and 

 they continue to advance in the same direction as the wind, so long as 



