THE TIDES 129 



Admirai Mottez in tlie Atlantic Océan a little ,north of the 

 Equator in 28° W. Long. 



Although there is a definite relationship between the 

 period, velocity, and length of a wave, there is none between 

 thèse and the height. The longest waves are usually met with 

 in the South Pacific, where their lengths vary from 600 to 1,000 

 feet and their periods from 11 to 14 seconds. Waves of from 

 500 to 600 feet in length are sometimes met in the Atlantic, but 

 the usual lengths are from 160 to 320 feet, and the periods 

 from 6 to 8 seconds. As to the heights of waves there is much 

 conflicting évidence in the records. Dumont D'Urville has 

 recorded a wave 100 feet high in 1837 off the Cape of Good 

 Hope. This vv^as an estimate, not a measurement, and although 

 many seafarers will agrée with D'Urville, the estimate is 



FiG. 22. — Erroneous Estimation of Height of Wave 



ON PiTCHING VeSSEL. 



probably too high. French marine officers measured many 

 waves according to instructions given by Arago. The highest 

 measured were in February, 1841, near the Azores, when from 

 42 to 50 feet was recorded. In the enclosed seas the height of 

 waves is much less. Probably in the North Sea waves never 

 exceed 31 feet in height, with periodicity of 9 seconds and 

 wave-length 147 feet (as maxima). 



There is considérable spéculation amongst fishermen and 

 seafaring men as to which, if any, of a group of waves is the 

 highest. This idea was known to the ancient Greeks, who 

 wrote of groups of three waves as being the highest in a sea- 

 way. Vaughan Cornish describes groups of three waves as 

 being higher than the rest in stormy weather in the North 

 Atlantic, and he claims to hâve followed them with the eye 

 for more than a mile when standing on the steamer's bridge. 



9 



