Much of our evidence of Greek and Roman knowledge of the 

 tides comes from Strabo (born c. 63 B.C.) who, in his Geography, 

 referred to the observations and writings of men before him. 

 Pytheas, who sailed to Britain and the North Sea around 325 b.c, 

 is credited with making the iirst systematic observations of the 

 tides, and being the first to relate the alternation of spring and 

 neap tides to phases of the Moon. Strabo knew that in certain 

 places the morning tides on successive days tended to resemble one 

 another more closely than morning and afternoon tides of the same 

 day (known as diurnal inequality). He also described the dangers 

 of tidal bores which regularly and dramatically invade certain rivers. 

 Pliny the Elder (a.d. 23 — 79), in his Natural History, correctly 

 explained that spring and neap tides are raised by the combined 

 effects associated with the Sun and Moon; '-and that diurnal ine- 

 quality is related to their positions north or south of the Equator. 



The tides vary greatly from place to place, often within a sur- 

 prisingly short distance. Only forty miles separate the Atlantic and 

 Pacific oceans along the Panama Canal, yet at the Atlantic end the 

 tidal range is about two feet while at the Pacific end it is about 

 fourteen feet. Around the British Isles the tidal range is greatest 

 in the Bristol Channel where, at Avonmouth, the spring range 

 reaches forty feet. At Portland Bill, near the center of the English 

 Channel coast, the spring range is only seven feet. The largest 

 range in the world is at the head of the Bay of Fundy, between 

 New Brunswick and Nova Scotia, where the spring range reaches 

 fifty feet at the head of the Minas Basin. Yet a few hundred miles 

 south, around Cape Cod, the range is less than three feet, although 

 it increases again farther south along the United States coast. 



As tidal ranges vary from place to place, so do tidal rhythms. 

 Along the shores of the Gulf of Mexico, where the range is no 

 more than a foot or two, the rhythm of the tides would seem 

 remarkable to anyone accustomed to two high waters and two 

 low waters a day. Here the tides are diurnal, with only one high 

 water and one low water each lunar day of twenty-four hours and 

 fifty minutes. On the other hand, the Pacific coasts of North Ameri- 

 ca have mixed tic^es — two highs and two lows a day, but usually 

 of markedly unequal heights. In fact, the tides everywhere are made 

 up of the oscillations of both diurnal and semidiurnal periods. 

 Where the semidiurnal oscillations predominate, as around the 

 European coasts, there are two tides a day. In areas such as the 

 Gulf of Mexico the semidiurnal oscillations are much weaker, and 

 it is the diurnal ones that govern the rhythms of the tides. 



The simplest way to measure the rise and fall of the tide is to 

 erect a vertical pole marked in feet, and then watch the change in 

 water level at regular intervals of time. But a more common practice 

 nowadays is to use a tide gauge which records the height of water 

 automatically (see diagram). The coastal tide gauge gives the 

 oceanographer a "cardiogram," as it were, of tidal height measured 

 against time. In principle, gauges used in the open sea are pressure 

 gauges that are laid on the bottom and record the changing pressure 

 of the water brought about by the changing depth. With their 

 positions marked by buoys, these gauges are left at sea for several 

 weeks and give oceanographers a continuous record of tidal change. 



Since Greek and Roman times men have known that the rhythm 

 of the tides was somehow related to the apparent motion of the 



Tide gauges located at various points 

 along a coast provide records which can 

 be analyzed to form a basis for predicting 

 future tides. The gauge shown here 

 consists of a float which transmits the 

 rise and fall of the water to the paper 

 on a recording drum. Above, a U. S. 

 Coast Guard officer reads a portable 

 tide gauge. 



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