VAEIETY IN TIDES MARMER 187 



moon is at its semimonthly maximum north or south declination, and 

 the latter being greatest when the moon is over the Equator. It is 

 in this varying relation of the magnitudes of the daily and semidaily 

 tide-producing forces through a fortnight that we find explanation 

 of tides which part of the time are of the mixed type and part of the 

 time of the daily type. 



The strikingly different characteristics of the varieties of tides 

 discussed above have been traced back to the combination of daily 

 and semidaily constituent tides of different times and ranges. The 

 question that immediately arises is, why do the waters of the sea in 

 different places respond differently to the tide-producing forces 

 of sun and moon? For these tide-producing forces are distributed 

 in a regular manner over the entire earth. 



To answer this question it is necessary to consider the physics of 

 the movement of bodies of water under the impulse of periodic 

 forces. Briefly, it may be stated that a body of water has a natural 

 period of oscillation which depends on the length and depth of the 

 body of water. Furthermore, when disturbed by periodic forces 

 that tend to upset its equilibrium, a body of water will respond best 

 to that force the period of which most closely approximates to its 

 natural period of oscillation. 



The principal tide-producing forces of sun and moon, as has been 

 noted before, are those having periods of half a day and a day, 

 respectively. As these tide-producing forces sweep over the earth 

 they put into oscillation the waters of the various oceanic 

 basins. But the response of any given oceanic basin to these 

 forces depends on the natural period of oscillation, which period 

 is determined by the length and depth of the basin. Those basins 

 whose natural periods of oscillation approximate to half a day 

 respond best to the semidaily tide-producing forces; hence the 

 semidaily constituent of the tide will predominate and the tide 

 in these basins will be of the semidaily type. Those bodies of water 

 whose natural periods of oscillation approximate to a day will re- 

 spond best to the daily tide-producing forces and thus give rise to 

 daily tides. At the same time, those bodies of water, the natural 

 periods of oscillation of which approximate to the daily forces as 

 closely as to the semidaily forces, will respond in approximately 

 equal degree to both kinds of tide-producing forces and hence give 

 rise to mixed tides. 



The varieties of tides described above are those most frequently 

 met with in the large world ports. There are places, however, 

 where local hydrographic features give rise to peculiarities not found 

 at other places. Along the open sea and in coastal bodies of water, 

 the durations of rise and fall of tide are approximately equal. This 

 gives the rising and falling portions of the tide curve a symmetrical 



