HARMONIC ANALYSIS’ AND PREDICTION OF TIDES 139 
400. Hour-marking device —The arm for the datum-line pen is se- 
cured 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. 
26). A spring keeps the armatures at equal distances from their re- 
spective 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. 25). The ivory disk 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. 
401. 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. 23) is in contact with the fixed platinum index (12); 
that is to say, at the times of high and low waters. When this con- 
tact 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. 24) just above the hand-crank 
shaft permits the cutting out of the current from the two electromag- 
nets. 
402. Adjustment of machine-—The adjustment of the machine 
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. 
403. Height-chain adjustment.—All amplitudes should be set at zero, 
so that the turning of each constituent 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 
constituent with a very small amplitude and operating the machine 
with the hand crank, note whether the index of 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. 
404. 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 constituent 
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. 
405. 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- 
