TIDES 



5808 



TIDES 



1777, but was later abandoned. The ruins of 

 the barracks and fortifications are still to be 

 seen. 



TIDES, the periodic rise and fall of ocean 

 waters, observed on most seacoasts twice a day. 

 Centuries ago, before many of the familiar 

 facts of astronomy and physical geography were 

 known, these daily movements of the ocean 

 were watched and studied, and the reason for 

 the phenomenon was eagerly sought by the wise 

 men. It was a problem, however, that baffled 

 philosophers until the seventeenth century, when 

 Sir Isaac Newton furnished the key to the solu- 

 tion with his theory of gravitation. According 

 to this principle, every particle of matter in the 

 universe during every instant of time is attract- 

 ing every other particle. Tides are caused by 



body. When the sun and moon are pulling in 

 the same straight line, as at full moon or new 

 moon, the effects of the solar high tide are 

 added to those of the lunar high tide, and the 

 result is a tide higher than the average. Such 

 waves are called spring tides. At the same time 

 there is a corresponding low ebb tide. When 

 the sun and moon act at right angles, the solar 

 high tide occurs at the same point as the lunar 

 low tide and prevents the latter from being as 

 low as usual; the solar low tide, on the other 

 hand, lessens the force of the lunar high tide. 

 These moderately high and low tides are called 

 neap tides. 



As has been stated, the tidal waves in the 

 open ocean are hardly noticeable, for in mid- 

 ocean there is a difference of only about three 



Moon 



the attractive force of the. sun 

 and moon as it is exerted on 

 the earth. That is, these 

 bodies pull upon the earth in 

 such a way as to cause a piling 

 up of the water envelope; the 

 effect on the solid earth is, of 

 course, not discernible, for the 

 land envelope acts as a rigid 

 body and does not perceptibly 



MoonQ 



CAUSE OP TIDES 

 (1) The attraction of the sun 

 and moon in conjunction; (2) 

 sun and moon in opposition ; 

 (3) moon in quadrature. 



yield to the pulling force, although the force is 

 present. It is as constant and unalterable as the 

 force of gravitation. 



The course of the daily tide is as follows: 

 There is a gradual rise for about six hours, and 

 then the water remains stationary for a short 

 time. Then it begins to recede and continues 

 to fall for the succeeding six hours. The high- 

 est stage is called high tide, and the lowest, low 

 tide. The landward movement is called flood 

 tide, and the receding one, ebb tide. These 

 terms have been applied to the wave move- 

 ments as observed on shores, for it is difficult 

 to distinguish the tides out in the open sea. On 

 every portion of a coast there are two high and 

 two low tides within a little more than a solar 

 day, the interval between two successive high 

 tides being about twelve hours and twenty-six 

 minutes. At different times in the year the 

 hours of high and low tides vary slightly. 



In respect to the formation of tides the moon 

 is more important than the sun, because it is 

 so much nearer the earth than is the larger 



feet between high water and 

 low water. On seacoasts the 

 height of the tides varies 

 greatly, because of the varying 

 shape of the shores. When the 

 tidal wave enters a wide- 

 mouthed channel or estuary 

 converging inland, the waters 

 pile up as they pass the nar- 

 rowing shores, and a rushing 

 stream called a bore is formed. In the Bay of 

 Fundy the tidal wave sometimes reaches a 

 height of sixty feet. Bores also occur in the 

 Amazon, the Ganges, in numerous estuaries of 

 the British Isles, and in other places. Such 

 tides are often very dangerous to sailing ves- 

 sels. Another phenomenon of tide waves is the 

 tidal race, formed when the tide rises to dif- 

 ferent heights in two bodies of water joined by 

 a narrow channel. Sometimes the force of such 

 a current renders navigation very dangerous. 



Tides are of value in keeping the otherwise 

 stagnant waters of bays and harbors in move- 

 ment. Large steamers usually leave port at 

 high tide; some landlocked harbors have so 

 shallow an entrance at low tide that all vessels 

 must wait for the flood before they can 'pass 

 over the bar. Mariners' charts, which show the 

 variations in tide for all harbors, are compiled 

 for the convenience of navigators and these are 

 carried by all tide-water vessels. E.B.P. 



Consult Wheeler's Practical Manual of Tides 

 and Waves; Ball's Time and Tide. 



