HARMONIC ANALYSIS! AND PREDICTION OF TIDES 141 
All. To set constituent angles.—After the amplitudes have been set 
and checked on both sides of the machine the angles are set for the 
beginning of the period of predictions, these settings being given in 
Form 445. The angles may be set from either side of the machine, 
except for constituents Sa and Ssa, for which there are no dials on 
the time side, as the readings are the same for both sides. As each 
constituent angle is set its releasable gear is clamped to the main 
vertical shaft. After all the angles have been thus set the three main 
vertical shafts must be unclamped to permit them to turn. 
412. Changing height scale-—There are three interchangeable height 
scales, known as the 40-foot, the 20-foot, and the 10-foot scale. The 
40-foot ring may also be conveniently used as a 4-foot scale. The 
scale to be used for any station is indicated in Form 445. In removing 
a scale from the machine a small button at the top is turned to release 
the ring, which is then lifted slightly as it is being removed. The 
desired scale is then placed on the machine and secured in place by a 
button. Before removing or replacing the height scale it is desirable 
that the height pointer be set approximately 45° to the left of its 
zero position in order to interfere least with the removal or replacement 
of the scale. 
413. The datum or plane of reference-——The hand-operating crank 
should be turned forward or backward until the index of the summa- 
tion wheel on the height side of the machine indicates mean sea level. 
It must be kept in mind, however, that as the index lines may come 
in conjunction at each complete rotation of the summation wheel 
there is a possibility of being misled in regard to the mean sea-level 
position. When in doubt, the operating crank should be turned 
forward to obtain a number of conjunctions, the corresponding height 
dial reading for each being noted. The conjunction that corresponds 
most closely with the average of such height readings will be the one 
that applies to the true zero position. Each complete turn of the 
height summation wheel will cause a change in the height reading of 
12 units, 6 units, or 3 units, respectively, according to whether the 
40-unit, 20-unit, or 10-unit dial is used. The height hand, which can 
be released by the milled nut on the face of the machine, may now 
be set to the scale reading that corresponds to the height of mean sea 
level above the datum which has been adopted for the predictions, 
this value being given in Form 445. 
414. The marigram gear.—There are three gear combinations, desig- 
nated as the 1:1, 3:2, and 2:1 ratios. The gear ratio to be used for 
any station is indicated in Form 445. When it is necessary to change 
the gear ratio, the machine should be first turned to its mean sea- 
level position. The change is then effected by sliding the lower set 
of gears horizontally, being careful to hold the upper set with one 
hand to prevent it from turning when the gears are released. Before 
engaging the gears in their new ratios the counterpoise for the pen 
carriage should be brought to a position approximately midway 
between the limits of its range of motion. The 1:1 ratio is obtained 
by sliding the lower set of gears as far as possible toward the height 
side of the machine, thus engaging the innermost gears; the 3:2 ratio 
by moving these gears toward the time side until the outer gears are 
engaged, and the 2:1 ratio by engaging the middle gear of each set. 
