54 THE OCEAN. 



same time very lilgli and very regular, and blows for a long time 

 from the same point of the compass. 



As to the width of the waves, that is to say, their total breadth 

 from base to base, observers have not obtained the same results ; but 

 there are few among them who have found the vertical height of the 

 crest of the wave to be less than a twentieth or more than a tenth 

 of the width. On an averap'e the heij^ht of an undulation of the 

 water is only equal to the fifteenth part of its base ; thus a wave 

 of 4 feet in height measures 40 feet from valley to valley, and a 

 wave 33 feet high is 495 feet in width. This is a much smaller 



Fig. 20.— Average Amplitude of Waves. 



size than would be imagined by the sailor lost in the midst of the 

 billows, which he sees rising around him in every direction. More- 

 over, the inclination of the waves varies with the force of the wind, 

 and the movements of the secondary undulations which intersect 

 the principal ones. 



The speed of the waves is only an apparent speed, like that of the 

 folds of a cloth, raised by a current of air. Thus, although the water 

 pressed by the wind rises and sinks by turns, it nevertheless hardly 

 changes its place, and objects floating on its surface move but slowly 

 and in an undulatory manner. The real movement of the sea is that 

 of a drifting current which gradually forms under the prolonged 

 action of the wind : but this general movement of the liquid mass is 

 after all inconsiderable. The only part which advances with the 

 storm is the foaming crest which, curling over the summit of the 

 waves, dashes down the slope in front. By their incessant move- 

 ments, the surface of the waves gradually increase in temperature, 

 as has been observed after a succession of violent storms.* 



The apparent displacement of the billows (which is rather difficult 

 to measure with exactitude in the open sea) varies in a regular 

 manner, according to the magnitude of its waves and the depth of 

 its waters. Thus according to the calculations of the astronomer 

 Airey, every wave of 100 feet in width, traversing a sea of 164 

 fathoms mean depth, has a velocity of nearly 2100 feet per second, 



* Joule. Cialdi, Sul moto ondoso del mare, p. 218. 



