136 U. S, COAST AND GEODETIC SURVEY. 



To set component 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 components Sa and Ssa, for which there are no dials on 

 the time side, as the readings are the same for both sides. As each 

 component 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 undamped to permit them to turn. 



Changing Jieight 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 remov- 

 ing 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. 



The datum or plane of reference. — The hand-operating crank should 

 be turned forward or backward until the index of the summation 

 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 corre- 

 sponds 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 to 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 cor- 

 responds to the height of mean sea level above the datum which has 

 been adopted for the predictions, this value being given in Form 445. 



The marigram gear. — There are three gear combinations, designated 

 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. 



In setting up the machine for successive stations there is a me- 

 chanical advantage in making the necessary gear changes before 



