134 U. S. COAST AND GEODETIC SUEVEY. 



Hour-marking device. — The arm for the datum-line pen is secured 

 to the outer end of a shaft which carries two armatures, one for the 

 upper and the other for the lower of two electromagnets {17 , fig. 17). 

 A spring keeps the armatures at equal distances from their respective 

 electromagnets. The upper electromagnet is designed for indicating 

 the hours on the datum line and is in a circuit that is opened and 

 closed by a platinum-tipped contact spring resting upon the edge 

 of an ivory disk in which are embedded, equally spaced, 24 narrow 

 strips of platinum {32, fig. 16). The ivory disl^ is mounted on the 

 shaft of the hour pointer, and as this rotates the platinum strips 

 successively make an electric contact that throws the datum-line 

 pen downward for an instant, making a corresponding jog in the datum 

 line, the downward stroke of the pen indicating the exact hour. 

 An extra strip of platinum placed close to the one representing the 

 midnight hour causes a double jog for the beginning of each day, the 

 downward stroke of the second jog indicating the zero hour. 



High, and low water marking device. — The lower electromagnet is in 

 a circuit that is closed when the platinum index on the time chain 

 {11, fig. 12) is in contact with the fixed platinum index {12) ; that is 

 to say, at the times of high and low waters. When this contact is 

 made, the electromagnet attracts the armature, which throws the 

 datum -line pen upward, causing a corresponding upward jog in the 

 datum line, and thus automatically marking the time of the high or 

 low water. 



A small switch {21, fig. 15) just above the hand-crank shaft permits 

 the cutting out of the current from the two electromagnets. 



Adjustment of machine. — The adjustment of the matchine should 

 be tested at least once each year and at any other time when there 

 is any reason for believing that a change may have taken place. The 

 following adjustments are required. 



Height-chain adjustment. — All amplitudes should be set at zero, so 

 that the turning of each component crank shaft will produce no motion 

 in the height chain. This should bring the summation wheel to its 

 zero position, but on account of a certain amount of backlash and 

 flexures in the machine this wheel may not be in an exact zero position 

 even when the chain is in adjustment. Now, set a single component 

 with a very small amplitude and operating the machine with the 

 hand crank, note whether the index ol the summation wheel oscillates 

 equal distances on both sides of its zero position. If not, the chain 

 should be adjusted by the adjusting nut at its fixed end at the back 

 part of the machine. 



Time-chain adjustment. — The adjustment of the time chain is 

 similar to that of the height chain. The zero position is indicated by 

 the conjunction of a small triangular-shaped index on the chain and 

 a fixed platinum index in the middle of the horizontal opening in the 

 dial face. A small amplitude being set on one of the component time 

 cranks and the machine operated by the hand crank, the chain index 

 should oscillate equal distances on both sides of the platinum point. 

 If it does not, the necessary, adjustment may be made at the fixed 

 end of the chain. 



Hour-hand adjustment. — This must be so adjusted that it will 

 register the exact hour at the same instant the circuit for the electro- 

 magnet is closed for the hour mark on the marigram, which is indi- 

 cated by a downward stroke of the datum-line pen. It is also neces- 



