112 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



The range of tide in the daily type also is 

 subject to a slow change of 18.6 years, due to 

 changes in the inclination of the moon's orbit 

 to the equator, as is the semidaily type, but is 

 opposite in phase. During the years when the 

 inclination of the orbit to the equator is greatest, 

 the range in the semidaily type is less than the 

 average, but in the daily type it is greater than 

 the average, and during the years when the 

 inclination is least, the range in the semidaily 

 type is greater than the average while in the 

 daily type it is less than the average. The results 

 from 19 years of observations are shown in figure 

 31 for Miami Beach and Pensacola, illustrating 

 the diflferences in the semidaily and daily types 

 as regards this 19-year variation. 



1935 



1940 



1945 



2.4 — 



FEET 

 1.4 — 



1.3 — 



1.2 — 



Figure 31. — Variation in yearly range of tide, Miami 

 Beach and Pensacola. 



The circles in figure 31 represent the yearly 

 values of the range of the tide as derived from the 

 continuous observations at the two stations, the 

 more or less smooth curves being drawn to follow 

 the yearly values with due consideration to the 

 years of maxima and minima which are fixed from 



theoretical considerations. The deviations of the 

 yearly values from the curves may be regarded as 

 irregularities due to wiad and weather. 



For Miami, the change in range from the mini- 

 mum in 1931-32 to the maximum in 1941 is 0.20 

 foot or 0.1 foot each way from the mean range 

 which represents a change of 4 percent from the 

 mean value of the range of 2.51 feet. For Pensa- 

 cola, the change in range from the minimum to the 

 maximum yearly value is 0.27 foot or 0.135 foot 

 each way from the mean range which represents 

 a change of 11 percent either way from the mean 

 value of the range of 1.26 feet. In the daily 

 type, therefore, the percentage change in the 

 yearly range of tide is greater than in the semidaily 

 type. 



MIXED TYPES 



From the preceding discussion of the semidaily 

 and daily types it is clear that the mixed types of 

 tide will vary in characteristics depending on the 

 ratios of the semidaily and daily constituents. 

 In the mixed semidaily type the diurnal inequality 

 becomes a salient feature necessitating the differ- 

 entiation between higher high water and lower 

 high water and between higher low water and 

 lower low water. The diurnal inequality, that is, 

 the difference between the two high waters and 

 the two low waters of a day will be greatest near 

 the times of the moon's semimonthly maximum 

 declinations and least when the moon is close to 

 the equator. During the 19-year cycle there may 

 be periods, in the years of maximum inclination 

 of the moon's orbit, when the tide will become 

 diurnal. 



In the mixed daily tides there will be more days 

 of the occurrence of two high waters and two low 

 waters each fortnight near the times of the moon's 

 crossing of the equator in the years of minimum 

 inclination of the moon's orbit. 



The changes from day to day in the features of 

 the tide of the mixed types of tide are thus much 

 more varied than in either of the simpler types. 

 However, the tide tables permit the delineation 

 of these changes from day to day. If the tide 

 tables give the daily predictions for the place 

 where this information is desired, the information 

 is immediately available. Otherwise, it is neces- 

 sary to infer the predictions from the predictions 

 at some port with similar tides. 



