SUN, MOON, AND TIDES — COLBERT 199 



mately. The mariner was given the information on the nautical 

 chart in the form of a tidal note. 



In more recent years the need for precise prediction of the tide be- 

 came important. Knowledge of exact tide conditions and the ebb and 

 flow of tidal currents at definite periods and hours was needed months 

 in advance for engineering purposes and for coastwise and trans- 

 oceanic commerce. As contrasted with the general and incomplete 

 information on the early charts, there is now printed in tide tables 

 accurate and detailed data on the time and heights of high and low 

 water. 



By the exchange of information with other maritime countries, tide 

 tables are printed for ports and seacoasts of all oceans. A casual 

 glance through these tables would show the great variation in the 

 tide for different geographic locations. 



For preparation of the tide tables, long and tedious computations 

 are avoided by the use of computing machines, such as the Coast and 

 Geodetic Survey tide-predicting machine designed and built in the 

 bureau under the direction of Dr. Harris and E. G. Fischer. Along 

 each side of the machine are set the harmonic constants for a given 

 seaport. Derivatives of 37 component curves may be set on a series 

 of pulleys over which a flexible chain passes to sum up their respective 

 movements. The machine carries out four operations simultaneously, 

 namely, (1) draws the curve of the predicted tide, (2) marks on the 

 curve the time of each high and low water, (3) indicates the time and 

 heights of high and low water on dials, and (4) exhibits on these dials 

 the height of the tide at any desired time. 



The first predicting machine was devised by Lord Kelvin, who ap- 

 plied the harmonic analysis to the problem of predicting the tide. 

 Complexities in this problem are caused partly by the varying configu- 

 rations of the shores and hydrographic features of different bodies of 

 water, but principally by variations in the combined effects of the 

 tide-producing forces of the sun and moon. The orbits of the sun 

 and moon are elliptical ; they are inclined to the plane of the earth's 

 Equator and the periods of revolution differ for these celestial 

 bodies. By the method of harmonic analysis are computed the am- 

 plitudes and phases of simple tides such as would be produced by a 

 number of suns and moons revolving in simple circular orbits, all in 

 the plane of the Equator. The complicated effects of the tide-pro- 

 ducing forces of sun and moon are broken down into a sufficient num- 

 ber of component curves to take care of the variations of the true sun 

 and moon from such simple circular movements. 



SOLAR TIDES 



One frequently hears the expression "the tide follows the moon/' 

 We have noted that the nearness of the moon overcomes the much 



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