president's addkess. 623 



wave is, in fact, in equilibrium with the wind when once the 

 height corresponding with the wind velocity is reached. It is a 

 matter of common observation that a certain breadth of ocean is 

 required before the wind can raise waves corresponding in height 

 to its velocity; this is technically known as "fetch." A steady 

 wave has a maximum length — distance from crest to crest— of 

 somewhere about twenty times its own height, and, in a general 



I 

 Diagram showing the relation of Wave-height to Wave-length. 



way, the " fetch " necessary for the production of a normal 

 wave is about 2,000 times its length. Taking the case before 

 cited of a wave 12 J feet high, which, we have seen, corresponds 

 to a wind velocity of 25 miles per hour : — 12J x 20 gives 250 feet 

 as the normal length of such a wave, and 250 x 2,000 = 500,000 

 feet, or nearly 95 miles. We see then that a wind blowing at 

 the rate of 25 miles per hour, requires a stretch of some 95 miles 

 of ocean in which to raise 12 J feet waves, the normal height for 

 this wind speed. Higher wind velocities require correspondingly 

 longer " fetches " of ocean surface in order to produce their 

 maximum wave efiPect. When the wind dies away the waves 

 tend to flatten down and form ocean swell, which really consists 

 of waves of comparatively small height, but considerable wave 

 length. With such a swell already in existence a fresh storm 

 travelling in the same direction will quickly raise waves of the 

 maximum height. Further, the rate at which wave motion 

 travels on water varies with the other properties of the wave. 

 Another important property of any wave is its frequency or 

 number of recurrences in a unit of time. This is a product of 

 ^^Mw and becomes smaller as the wave becomes larger. In 



length ^ 



