34 HARBOUR CONSTRUCTION. 



Ground-swell undulations are sometimes very long as com- 

 pared with their height, and they "feel" the bottom at great 

 depths. The mid-ocean storms by which these rollers are generated 

 often do not blow home. To this class of waves the well-known 

 rollers of the Cape of Good Hope belong. 



The veering or backing of the wind, to the extent of a point 

 or two during a storm, causes a " lumpy " sea ; and a shift in the 

 direction of the wind, to any considerable extent, tends to reduce 

 the height of waves ; therefore the duration of a storm, steadily 

 in any given direction, is an important factor in the raising of a 

 heavy sea. 



In support of the foregoing, I refer the reader to the records 

 of gales given on p. 37, et seq. 



A self-recording tide-gauge, fully exposed to the open ocean, 

 but so arranged as not to be affected by ordinary passing waves, 

 will plot a continuous diagram of the tidal wave, which will often 

 be found to have by no means so regular and flowing a line as 

 might have been expected. On the calmest days, undulations 

 of several inches in height, and having varying but usually very 

 long periods, will be represented in the diagrams. 



A careful study of these will show that the ocean is being 

 traversed by several sets of undulations. At certain periods 

 undulations larger than the others will be found, they being, 

 doubtless, equal to the sum of the volumes of the waves, which 

 at these periods coincide. 



Such undulations as those to which I now refer are not 

 discernible on the surface of the water, so that their existence 

 would not be suspected but for the sensitive instruments by 

 which they are recorded. These different series of undulations 

 may either be travelling in the same direction with different 

 periods, or in different directions with equal or unequal periods. 



I quote the following from Reid's " Law of Storms," chap iv., 

 respecting the direction of swell raised by storms. 



"Whilst sailing on the borders of the tropic in the Northern 

 Atlantic, I have frequently watched the gradual change in the 

 direction of the swell supposed to proceed from distant gales of wind, 

 and it seemed to change in conformity to the usual progressive track 

 of storms. 



" When living in the Bermuda Islands, I was frequently interested 

 by observing the change of direction in the surf which beat against 

 their shores. A coming storm would roll its undulations so as to 



