THE IMPENETRABLE SEA 



Seamen have a rough and ready method of forecasting 

 the size of the waves which may be expected under gale 

 conditions. They call the uninterrupted distance over 

 which a wind has been building up the 'Tetch". Short 

 fetches produce small waves — long fetches can produce 

 enormous ones. Taking the square root of the fetch 

 (measured in nautical miles) seamen multiply it by one 

 and a half, and this gives them, in feet, the height of the 

 waves which the wind is building up across the fetch. 

 This simple formula proves remarkably rehable in fore- 

 casting the approaching wave heights on widely varying 

 stretches of water — even up to fetches several hundreds of 

 miles in extent. 



Those who go down to the sea in ships have many 

 other methods of estimating the height and force of 

 waves. So with inland waterways. Canal navigators 

 know that there is a certain speed at which a canal boat 

 may be propelled, for it rides on a wave and is carried 

 forward by it, and if a calculation is made which 

 takes all factors into consideration the speed can be 

 maintained with the least additional expenditure of 

 power. 



The mathematical formulae covering wave motion — 

 whether in aeroform bodies, in solid bodies, or in liquids 

 — are vastly complex, but there is a simple, straight- 

 forward relationship between sea waves and their speed 

 which is not hard to understand. In this relationship the 

 height of a wave, strangely enough, does not come into it. 

 The formula is simply this : That the speed (in feet per 

 second) of any wave is equal to the wave-length (the 

 distance from one wave-crest to the next) divided by the 

 time that elapses (in seconds) between successive waves 

 as they pass any fixed point. Using this formula — which 

 is easier to apply in practice than might appear as you 

 read it — you can calculate the speed of any wave when 

 you are by the sea. 



The infinitely complicated movements of the sea's 



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