TIDAL OBSERVATIONS 589 



The least and greatest lunitidal intervals due to priming and lagging of the tides may be 

 designated as Prime HWI, Prime LWI. Lag HWI. and Lag LWI. They may be derived 

 from the mean lunitidal intervals by the following formulas : 



PrimeHWI = HWI— 127 (S,--MJ (15) 



Lag HWI = HWI+i27 (S/--M,) (16) 



Substituting LWI for HWI in (15) and (16) gives the corresponding values for low 



water. The values of S,^M, are given in (7), which, being substituted in the above 



equations, together with the values in (3) and (4), gives : 



Cape Flora Teplitz Bay 



T, . TTTTTT h m h m 



Prime HWI 9 02.2 5 21.7 (17) 



Lag HWI 10 26.8 7 05.5 (18) 



Prime LWI 2 55.0 II 31.9 (19) 



^^g ^WI 4 19.6 13 15.7 (20) 



The declination of the moon also makes a change in the lunitidal intervals and heights of 

 the tide, which is usually greatest when the declination becomes a maximum, at which time 

 the moon is not far from the tropics. Hence the tides due to the moon's declination, when at 

 their most pronounced type, are called tropic tides. At the time of the tropic tides the two 

 high or two low waters of the same day are generally unequal, and the range from the higher 

 high water to the lower low water is called the great tropic range. 



The lunitidal intervals for the tropic tides may be obtained from the mean intervals by 

 the following equations : 



Tropic HHWI = HWI — 2.07 x value from Table 44 * (21) 



Tropic LHWI = HWI — 2.07 x value from Table 44 * (22) 



Tropic HLWI = LWI — 2.07 x value from Table 44 * (23) 



Tropic LLWI = LWI — 2.07 x value from Table 44 * (24) 



In Table 44 * of these equations the arguments are different for each phase of tide, the 

 corresponding intervals being as follows : 



Cape Flora Teplitz Bay 



h m h m 



Tropic HHWI 10 19.05 5 58.8a (25) 



Tropic LHWI 8 59.9 6 24.9 (26) 



Tropic HLWI 3 39.8 12 13.5 (27) 



Tropic LLWI 3 22.5^ 12 33.2a (28) 



The tropic intervals for the higher high water and for the lower low water are marked by 

 the letters a. and b in order to enable one to obtain the approximate time of these tides by adding 

 the interval to the upper or lower transit of the moon as explained below. When the tropic 

 interval (HHWI or LLWI) is marked a add the interval to the local time of the moon's 



lower *'^*°^^*' °^ meridian passage, for "q^j.^ declination of the moon ; and when it is marked b 

 add the interval to the local time of the moon's J^^g^ transit for ^°"(^i^ declination of the moon. 



The tropic tides may be said to be formed by the combination of a semidiurnal wave with a 

 diurnal wave. The tropic lunitidal interval of the diurnal wave may be found by the equation 



D.HWI = 0.0345 (K°, + O°0 a (29) 



where I), stands for diurnal. 



* See note, p. 588. 



