38 HARBOUR CONSTRUCTION. 



at Port Elizabeth, which extended over a period of nine years ; 

 and some of the oldest inhabitants told me that the waves were 

 as high on that occasion as any they remembered to have seen. 



At another time, in the year 1883, I measured unbroken 

 waves 13 feet in height, trough to crest, as they passed under 

 the same staging during a south-east gale which had been 

 blowing for about fifty consecutive hours, the maximum wind 

 velocity being 68 miles per hour equal to, say, 23 Ibs., pressure 

 per square foot and the depth of water and length of fetch as 

 before. 



Again, during a south-east gale which quickly followed the 

 foregoing, and which had been blowing for three days, the wind 

 velocity at the time of observation being 58 miles per hour, I 

 similarly measured unbroken passing waves having a height of 

 only 10 feet. Their length, crest to crest, was 200 feet, and 

 their velocity 20 feet per second. The depth of water was about 

 24 feet. 



It will be observed that the greatest wind velocities recorded 

 during these three gales were respectively 60, 68, and 58 miles 

 per hour ; whereas the greatest heights attained by the waves 

 on the three occasions were respectively 21 feet, 13 feet, and 

 10 feet, fetch, depth of water, etc., being in all cases practically 

 the same. 



There can be little doubt that this apparent anomaly was 

 due to either or both of the following causes, viz. a difference in 

 the range of the respective gales, or a variation in the direction 

 of their course possibly far from land. 



The wind velocities were automatically recorded by an 

 excellent anemograph, so they may be considered reliable, at all 

 events for purposes of comparison. 



I have given the results of these three different sets of 

 observations because I think they clearly show that, even in 

 gales of considerable duration, a given fetch, wind velocity, and 

 depth of water will not always produce weaves of uniform height, 

 such height depending greatly upon the varying conditions of 

 the gales producing the waves, both as regards their extent and 

 duration in one direction. 



The direction of gales, without doubt, often varies consider- 

 ably in a distance of 200 or 300 miles from the shore. 



This is one more instance illustrating the necessity of extend- 

 ing the period over which observations are taken as much as 



