48 HARBOUR CONSTRUCTION. 



effect of this barrier, in arresting the freest portion of the tidal 

 wave in the Southern Ocean, is very apparent by the crowding 

 together of the cotidal lines * in the hook which the east coast 

 of Patagonia forms north of Cape Horn. 



"Priming" and " Lagging" of Tides. At new and full moon, 

 the respective high and low waters of the lunar and solar tides 

 being superposed, the observed tide is, as will have been gathered, 

 a compound one. 



When these tides do not exactly coincide, the maximum 

 elevation and depression of the compound tide will lie some- 

 where between those of the tides of which it is composed, but 

 they will be closer to those of the lunar tide, by reason of its 

 greater magnitude. 



If, therefore, about the time of new or full moon, the sun 

 passes the meridian before the moon, the crest of the solar wave 

 is slightly in advance of that of the lunar or main wave, and 

 the tide is " primed," or accelerated. If, on the other hand, the 

 sun passes the meridian after the moon, the crest of the solar 

 wave lags behind that of the lunar wave, and the tide is said to 

 " lag," or be retarded. This explains why the periods between 

 successive observed tides are so irregular as they are, notwith- 

 standing that the lunar and solar tides, individually, recur at 

 equal intervals of time. 



The extent to which this retardation and acceleration takes 

 place does not exceed one hour, and it is greatest about 4J days 

 before and after spring tides. 



Tides affected by Variations in Distance of the Sun, Moon, and 

 Earth from each other ; also by Declination. Since the earth and 

 the moon revolve in elliptical orbits, their respective distances 

 from the sun, and from each other are constantly changing ; and 

 inasmuch as the power of attraction which bodies exert upon 

 one another varies, as already stated, inversely as the squares 

 of the distances between them, it follows that these variations 

 must be reflected in the tides. 



The effects thus produced are modified by the declination 

 of the sun and moon, and by the varying power of the sun's 

 attraction upon the moon. In short, every variation in the 

 relative positions of the sun, moon, and earth, whether recurring 

 at daily, monthly, or annual periods, or extending over a series 



1 These are contour lines of high water plotted at intervals of one hour, thus 

 showing the progress of the tidal wave over the ocean. 



