UNITED STATES COAST AND GEODETIC SURVElf TIDE- 

 PREDICTING MACHINE NO. 2. 



GENERAL STATEMENT. 



One of the important functions of the Coast and Geodetic Sur- 

 vey is the preparation and publication of a tidal. calendar, issued 

 annually, from one to two years in advance, under the name of 

 ''Tide Tables." These tables give to the mariner, the engineer, and 

 others interested, information as to the time and magnitude of the 

 tidal fluctuations of the sea. They show directly the predicted 

 time to the minute and the height to the nearest tenth of a foot of 

 every high and low water in the year at most of the principal seaports 

 of the world, and give indirectly, by means of an appended auxiliary 

 table, the same information for more than 3,000 intermediate ports 

 or stations. ♦ 



While the civil calendar deals only with the positions of the impor- 

 tant heavenly bodies, the preparation of the data for a tidal calendar 

 involves also a knowledge of the effect upon the sea due to the force 

 of attraction of the moon and sun. The laws governing the motj^n 

 of liquid bodies under the influence of the force of attraction are well 

 known; the tides upon a sphere covered completely by water of 

 uniform depth could be foretold from astronomical data alone. But 

 the continents, islands, and the great difference in the depths of the 

 oceans prevent such ideal tides, as regards both time and magnitude; 

 so that it is necessary to resort to observation in order to ascertain 

 the elements characteristic of each station, before the height and time 

 of future tides can be computed. 



These observations are made mainly by means of automatic- 

 recording tide gauges, in which the rise and fall of the water is made 

 to move a pencil back and forth across a strip of paper, the latter 

 being kept moving at imiform speed by clockwork, thus producing a 

 continuous tide curve. 



Owing to the extremely complicated nature of the motions of the 

 moon and earth it was practically impossible to take account of more 

 than the most important elements in predicting tides until there was 

 devised, about 1867, by Sir William Thomson (later Lord Kelvin) a 

 system for the reduction of tides known as '' harmonic analysis." 

 This system assumes, in the place of the apparent sun with varying 



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