AND OTHER WATER WAVES 97 



or darker, where the whole group of shorter waves 

 seems slightly raised, and this broad, heaved-up 

 piece seems to travel with great speed, like the 

 shadow of a scudding cloud. 



To take the example afforded us by Sir G. G. 

 Stokes' observation, let us think of the group of 

 waves which he observed as they were when the 

 wind first dropped, and think of them first apart 

 from the shorter waves with which they were then 

 really associated. They were not then a band 

 1,100 miles broad of almost perfectly harmonic 

 undulations with length from 866 feet to 1,481 

 feet, but a much narrower band in which the 

 water undulated with less regular motion, the 

 surface having a less regular form. I shall take 

 the average period of undulation of the water in 

 this band as 15 seconds, and the wave-length, 

 therefore, as 1,150 feet. This part of the total 

 wave-disturbance I call the swell, and I inquire, 

 What was the height of the swell? Now, the 

 height of the principal waves shorter than the swell, 

 i.e., the dominant, or storm waves, we know fairly 

 well from the preceding records. Most of them 

 were 30 feet high, with fairly frequent larger in- 

 dividuals of 40 feet, and some of them were not 

 much more than 20 feet high. 



The diagram shows how such a condition can 



