Jtides and currents in boston harbor 101 



ing very much alike. As the declination increases the difference in consecutive 

 ranges increases, morning and afternoon tides beginning to show decided 

 differences, and at the times of the moon's maximum semimonthly declination 

 these differences are very nearly at a maximum. But like the response to 

 changes in the moon's phase and parallax, there is a lag in the response to 

 the change in declination, this lag being known as the "age of diurnal inequality " 

 or "diurnal age." Like the phase and parallax ages, the diurnal age varies 

 from place to place, being generally about one day, but in some places it may 

 have a negative value. 



When the moon is on or close to the Equator and the difference between 

 morning and afternoon tides small, the tides are known as "equatorial tides". 

 At the times of the moon's maximum semimonthly declination, when the differ- 

 ences between morning and afternoon tides are at a maximum, the tides are 

 called "tropic tides," since the moon is then near one of the Tropics. 



The three variations in the range of the tide noted above are exhibited by the 

 tide the world over, but not everywhere to the same degree. In many regions 

 the variation from neaps to springs is the principal variation; in certain regions 

 it is the variation from apogee to perigee that is the principal variation; and 

 in other regions it is the variation from equatorial to tropic tides that is the 

 predominant variation. 



The month of the moon's phases (the synodical month) is approximately 

 29H days in length; the month of the moon's distance (the anomalistic month) 

 is approximately 27 J^ days in length; the month of the moon's declination (the 

 tropic month) is approximately 27J^ days in length. It follows, therefore, that 

 very consideralbe variation in the range of the tide occurs during a year due 

 to the changing relations of the three variations to each other. 



DIURNAL INEQUALITY 



The difference between morning and afternoon tides due to the declination of 

 the moon is known as diurnal inequality, and where the diurnal inequality is 

 considerable the rise and fall of the tide is affected to a very marked degree both 

 in time and in height. Figure B represents graphically the differences in the 

 tide at San Francisco on October 18 and 24, 1922. On the former date the moon 

 was over the Equator, while on the latter date the moon was at its maximum 

 south declination for the month. The upper diagram thus represents the equa- 

 torial tide for San Francisco, while the lower diagram represents the tropic tide. 



It will be noted that on October 18 the morning and afternoon tides show very 

 close resemblance. In both cases the rise from low water to high water and the 

 fall from high water to low water took place in approximately six hours. The 

 heights to which the two high waters attained were very nearly the same, and 

 likewise the depressions of the two low waters. 



On October 24, when the moon attained its extreme declination for the fort- 

 night, tropic tides occurred. The characteristics of the rise and fall of the tide 

 on that day differ markedly from those on the 18th, when the equatorial tides 

 occurred, these differences pertaining both to the time and the height. Instead 

 of approximately equal duration of rise and of fall of six hours, both morning 

 and afternoon, as was the case on the 18th, we now have the morning rise occupy- 

 ing less time than the afternoon rise and the morning fall more time than the 

 evening fall. Even more striking are the differences in extent of rise and fall of 

 morning and afternoon tides. The tide curve shows that there was a difference 

 of a foot in the two high waters of the 24th and a difference of almost 3 feet in 

 the low waters. 



Definite names have been given to each of the two high and two low waters 

 of a tidal day. Of the high waters, the higher is called the "higher high water" 

 and the lower the "lower high water." Likewise, of the two low waters of any 

 tidal day the lower is called "lower low water" and the higher "higher low water." 



The diurnal inequality may be related directly to the ratio of the tides brought 

 about, respectively, by the diurnal and semidiurnal tide-producing forces. Those 

 bodies of water which offer relativelj' little response to the diurnal forces will 

 exhibit but little diurnal inequality, while those bodies which oflFer relatively 

 considerable response to these diurnal forces will exhibit considerable diurnal 

 inequality. On the Atlantic coast of the United States there is relatively little 

 diurnal inequality, while on the Pacific coast there is considerable inequality. 



It is obvious that with increasing diurnal inequality the lower high water and 

 higher low water tend to become equal and merge. When this occurs there is 

 but one high and one low water in a tidal day instead of two. This occurs 

 frequently at Galveston, Tex., and at a number of other places. 



