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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



cola. In figure 24, are shown the tide curves for 

 those two places for the same 8 days of June 1948 

 as in figure 23. The vertical lines at the top of 

 figure 23 indicate noon of each day. 



Compared to the two Gulf coast stations shown 

 in figure 23 the tide curve at Key West resembles 

 that at Cedar Keys more than that at Pensacola. 

 Nevertheless, there are differences apart from that 

 of range. At Cedar Keys the differences between 

 the two low waters of a day are larger than those 

 between the high waters, while at Key West the 

 case is reversed. In fact, it will be noted in figure 

 24 that the lower high water at the beginning of 

 the curve for Key West did not rise to the height 

 of sea level. 



At Galveston for the first 4 days there was only 

 one high and one low water daily, as at Pensacola, 

 but the fall from high water during these days 

 was different in character from that at Pensacola, 

 and the period of two tides a day began several 

 days earlier than at Pensacola. 



We may take the record at Port Isabel, Texas, 

 and at Tampico, Mexico, to exemplify the tide on 

 the western shore of the Gulf. The tide curves at 

 the two places for the last 8 days of June 1948 are 

 shown in figure 25. The vertical lines at the top 

 of the figure indicate noon of each day. 



At Port Isabel the tide curve resembles that at 

 Pensacola closely except that at the latter place 

 there was only 1 day with two high and two low 

 waters, while at Port Isabel there were 3 days 

 during which two high and two low waters oc- 

 curred. At Tampico the tide cirrve appears to 

 be about halfway between the tides at Galveston 

 and at Port Isabel. 



To complete the circuit of the Gulf we may 

 discuss the tide at Coatzacoalcos and Progreso, 

 Mexico, on the south shore and at Habana, Cuba, 

 at the southeastern entrance. These are shown 

 in figure 26 for the last 8 days of June 1948. As in 

 figures 23-25, the vertical lines at the top of the 

 figure indicate noon of each day. 



At both Coatzacoalcos and Progreso the tide 

 resembles that at Tampico, but at Habana the 

 tide curve resembles most nearly that at Key 

 West (fig. 24) except that the differences between 

 the two high waters of the day are more intensified. 



In the cursory examination of the tide for several 

 days at 10 stations along the shores of the Gulf we 

 have found very decided differences even though 

 we have disregarded differences in range of tide. 



Differences in range of tide may be regarded as 

 differences of degree rather than of kind. The 

 differences we have found are of a more profound 

 character than differences in range; they are 

 differences of kind. Moreover, it must be borne 

 in mind that at any place the tide changes 

 in range from day to day, from month to 

 month, and from year to year, and as our 

 examination of the 8 days illustrated in figures 

 23 to 26 discloses, the character of rise and fall 

 changes from day to day also. These changes in 

 range and character of tide arise primarily from 

 the changing positions of the moon with relation 

 to earth and sun, but if we attempt to correlate 

 these changes with the position of the moon we 

 find the tides at the different places responding 

 in different fashion. 



Hence, if we are to avoid describing the tide at 

 any place by detailed exposition of numerous 

 changing features, we must make use of some 

 clarifying concept which will bring the various 

 tidai features into genetic relationship. Such a 

 clarification is furnished by the concept of type 

 of tide through the use of harmonic constants. 



HARMONIC CONSTANTS 



The tide-producing forces of sun and moon 

 change from day to day in accordance with the 

 changing positions of those bodies relative to 

 the earth, but by mathematical analysis the tide- 

 producing forces can be resolved into a number of 

 simple periodic forces each having a fixed period- 

 In other words, with regard to the tide-producing 

 forces we may conceive the actual sun and moon 

 with their complicated movements relative to the 

 earth to be replaced by a number of hypothetical 

 tide-producing bodies. These hypothetical tide- 

 producing bodies are conceived as moving in 

 cu-cular orbits about the earth in the plane of the 

 equator and each bringing forth a constituent tide 

 of its own period. 



The periods of these hypothetical tide-producing 

 bodies can be calculated with precision from 

 astronomical data. Hence, the actual tide at any 

 place may be conceived as being made up of a 

 number of constituent or component tides, each 

 with a fixed period or speed corresponding to its 

 appropriate hypothetical tide-producing body. 



Analysis shows that the principal lunar com- 

 ponent has a period of 12" 25"° or an angular 

 speed of 28.98° per hour. It is designated by the 



