28 UNITED STATES COAST AND GEODETIC SURVEY. 



For marking upon the sheet the record of the predictions indicated 

 to the operator by the pointers and dials there are provided two pens 

 (plates 7 and 14), one for tracing a base line and marking the hour and 

 day spaces and the times of the high and low waters, and the other 

 for tracing the predicted tide curve. The curve pen uses about half 

 the ink it can hold for a year's curve, the base-line pen somewhat less. 



Base-line pen. — The base-line pen (plates 7 and 14) is mounted by 

 means of a metal lock joint, like that of the curve pen, on a swivel 

 arm with a spring for pressing against the paper. The swivel arm is 

 secured to the outer end of a shaft (plate 14), which carries two arma- 

 tures, one for an upper and one for a lower electromagnet. A spring 

 keeps the armatures at equal distances from their respective electro- 

 magnets, the pen tracing a straight line upon the paper moved along 

 under it. The swivel arm holding the pen can be raised or lowered a 

 small amount for exact adjustment of the pen point to mean sea level 

 of the curve. The electromagnets, together with the pen shaft and 

 its armatures, are mounted upon an angle plate held against the inner 

 side of the front plate of the dial case by two screws which, when 

 loosened, permit of adjusting the pen point laterally, so that the curve 

 pen will just clear it in its vertical movement. The unavoidable 

 error in time, due to the fact that both pens can not pass through the 

 same point, amounting to two or three minutes, can at any time be 

 accurately recorded on the sheet by pulling the curve pen, when above 

 the base line, down and across the base line, thus making it trace a 

 vertical line, and then making a vertical line with the base-lino pen 

 by pressing against either of the armatures. The distance between 

 these two lines is the measure of the error. 



The upper electromagnet is in circuit with a battery and a current- 

 making device, which consists of a platinum-tipped contact spring 

 resting upon the edge of a rubber disk, in which are imbedded, accu- 

 rately spaced, 24 narrow strips of platinum. This rubber disk is 

 secured to the shaft of the hour pointer within the dial case (plate 9) . 

 An extra strip of platinum is placed close to that representing the 

 twenty-fourth hour, or midnight. The contact spring can be brought 

 to exS,ct agreement with the hour and minute pointers by means of a 

 fine-threaded screw. As each platinum strip passes under the contact 

 spring the upper electromagnet attracts for a moment the armature, 

 which throws the base-line pen downward for an instant and makes a 

 short mark below the base line for each dial hour, a double mark indi- 

 cating the twenty-fourth hour, or midnight. 



Another circuit, including the same battery but the lower electro- 

 magnet, is closed when the projecting point of the platinum link in 

 the time chain is in coincidence and contact with the fixed platinum 

 index seen by the operator in the middle of the horizontal opening in 



