ON TIDES AND TIDAL ACTION IN HARBORS. 209 



of tbese waves will remain far behind the verticals of the disturbing 

 bodies, Itecause the inertia and friction of the water prevent the rapid 

 change of form required, and because, although the elevating force is 

 greatest under the vertical, it still continues to act in the same direction 

 for some hours after the passage of the luminary, with but little 

 diminished Ibrce. 



This retardation, which would be sensible under the simple sui^posi- 

 tion of an uninterrupted ocean covering the earth's surface, becomes 

 very considerable under the actual circumstances of the case. The 

 depth of the sea varies so mucli, and the form of its basin, taken as a 

 whole, is so interrupted by the land, that no regular progressive move- 

 ment of the tide-wave can take place, except in the great Southern 

 Ocean. At all points on the coast the phases of the tide will follow 

 the periodicity of the forces causing them, but at each point, at a greater 

 or less interval from tbe culmination of the sun or moon, according to 

 its local position, and the more or less circuitous course taken by the 

 tide- wave to reach it. This interval and the actual rise and fall of the 

 tide must be determined for each place by special observation. 



LUNI-SOLAR PHASES OF THE TIDES. 



The close relations which the times of high water bear to the times 

 of the moon's passage show that the moon's influence in raising the 

 tides must be much greater than the sun's. In fact, while the ichole 

 attraction of the sun upon the earth far exceeds that of the moon, yet 

 owing to the greater proximity of the latter, the difference between 

 its attraction at the center of the earth and at the nearest or most 

 remote point of its surface, which difference alone produces the tides, 

 is about two and a half times as great as the difference of the sun's 

 attraction at the same points. 



SEMI-MONTHLY INEQUALITY. 



We will now consider the particular phases resulting from the com- 

 bination of the lunar and solar tides, and from tbe varying positions of 

 those bodies. There will be two complete lunar tides in every lunar 

 day of twenty-four hours fifty-two minutes, and also two complete 

 solar tides in every mean solar day of twenty-four hours. These are 

 known as the semi-diurnal tides, and constitute the i)rincipal variations 

 of the sea-level. The combined effect of these two fluctuations will be 

 most readily understood by reference to the annexed diagram, in which 

 the lunar tide is represented by dashes, the solar by dots, and the com- 

 bined or actual tide by a full line. At the time of syzygies, or full and 

 change of the moon, the effects of both sun and moon combine together 

 to produce the spring-tides, when high water is higher and low water is 

 lower than at mean tides by the amount of the solar tide. At quad- 

 ratures the high water of the sun will combine with the low water of 

 14 s 



