8 UNITED STATES COAST AND GEODETIC SURVEY. 



the Survey. It combines the essential principle of similar machines 

 that had previously been built and includes also a number of new 

 features. 



The. British tide machines merely trace a curve of the predicted 

 tides from which the high and low waters are scaled off and tabu- 

 lated for the printer. 



The Ferrel tide machine of the United States Coast and Geodetic 

 Survey gives the time and height of the tide on its face, but does 

 not trace any curve. 



The United States Coast and Geodetic Survey tide-predicting ma- 

 chine No. 2 combines both features, for the time and height of the 

 tide is shown upon dials, as in the Ferrel machine, and it traces a 

 curve similar to that of the British machines. 



The main part of the machine is in three sections, each consisting 

 of two hard-rolled brass plates, upright, parallel, and about 6 or 7 

 inches apart. These plates support a system of sliding frames, 

 shafts, gearing, and dials for the production and recording of the 

 harmonic motion that represents the several elementary components 

 of the tide. The sections are securely fastened to a heavy cast-iron 

 base, and the entire machine occupies a space about 1 1 feet long, 2 

 feet wide, and 6 feet high, and is surrounded by a glass case suffi- 

 ciently large to enable the operator to move around inside while 

 adjusting the maliine. 



The tide is assumed to be composed of many elementary com- 

 ponents, each of which is due to the mean motion of the moon or of 

 the sun or to some periodic variation in the motion of either. Each 

 component may be represented by a simple harmonic motion, or 

 graphically by a cosine curve which is defined by its amplitude and 

 its epoch, or diflPerence in time between the astronomical cause and 

 the following high water of the component. The amplitudes and 

 epochs of the components are known as harmonic constants and are 

 obtained from the actual tidal record for any station by analysis. 

 The harmonic constants for any one station remain practically the 

 same for all time, but each station has a different set of constants. 



The tide-predicting machine mechanically sums these elementary 

 components for the station for which predictions are being made. 

 The machine is designed to take account of 37 such components. 

 For each component there is a crank attached to a shaft that is 

 geared to rotate at a speed that is proportional to the actual speed 

 of the astronomical cause producing that component. Each crank 

 contains an adjustable pin, which may be set within certain limits 

 at any distance from the axis of rotation. Before beginning the pre- 

 dictions for any station these pins are all set in accordance with the 

 amplitudes of the components included in the tide at that station, 

 and the initial positions of the cranks are set in accordance with the 



