WA VES. 37 



actual observations, for calculating the height of waves during 

 heavy gales. 



Height of waves in feet = 1J times square root of fetch in 

 nautical miles. 



This, of course, is on the assumption that the depth of water 

 is sufficient for the full development of the waves, and that the 

 storm is of sufficiently long duration to raise them to the 

 maximum height of which the fetch will admit. 



For short fetches, Mr. Stevenson considers the following 

 formula more accurate : 



H = l-5VD + (2-5 - yS) 



H = height of waves in feet during strong gales. 



D = fetch, or length of exposure, in nautical miles. 



Mr. Hawksley has also constructed a formula for calculating 

 the height of waves, which is likewise based upon numerous 

 observations ; but the results obtained by applying it vary to a 

 very material extent from those which Mr. Stevenson's formula 

 gives. 



In view of the numerous conditions which affect the height 

 of waves, it seems doubtful if it is possible to construct any 

 reliable formula by which it can be calculated. 



In estimating the possible height of waves, whether by the use 

 of formulae or otherwise, it should be remembered that, under 

 conditions the most favourable to wave propagation and trans- 

 mission, the height of waves (trough to crest) is limited by and 

 does not exceed the depth of water which they are traversing, 

 and, further, that the maximum height of waves (region of Cape 

 Horn perhaps excepted) is generally admitted to be between 

 40 feet and 50 feet. Be this as it may, an engineer need not be 

 very solicitous about waves over 40 feet in height, because, in 

 designing a work to occupy a position where such waves could 

 assail it, he would not be likely to err by making it too strong. 



From a staging in Algoa Bay (Cape Colony) I have measured, 

 with a sounding chain and weight, unbroken passing waves of 

 21 feet in height, trough to crest, in water of 23 feet in depth. 

 These were set up by a gale having a wind velocity of about 

 60 miles per hour. The fetch was practically unlimited, being 

 open to the Indian Ocean, and the depth of water gradually 

 increased seaward at the rate of about 4 fathoms per mile. 

 This was as high a sea as any that occurred during my residence 



