HARMONIC ANALYSIS AND PREDICTION OF TIDES. 135 



sary that the zero hour or beginning of the day shall correspond to the 

 double hour mark on the marigram. This adjustment may be accom- 

 plished by moving the hour hand on its shaft after releasing its set 

 screw. A finer adjustment may be effected by changing the position 

 of the contact spring back of the dial face. 



Minute-hand adjustment. — This is to be adjusted to read zero on 

 the exact hour indicated by the hour hand and the closing of the 

 electric circuit for the hour mark. The adjustment may be accom- 

 plished either by moving the minute hand on its shaft after releasing 

 its set screw or by means of the releasable gears on the main vertical 

 shaft of the dial case. The adjustments just described are those which 

 need be made onh' occasionally. Other adjustments are taken into 

 account each time the machine is set for a station. 



Setting predicting machine. — The time indicators on the face of the 

 machine are first set to represent the exact beginning of the period 

 for wliich predictions are to be made, which will ususally be hour of 

 January 1 of some year. The hour and minute hands should always 

 be brought into place by the turning of the operating crank in order 

 that the adjustment of these hands relative to the electromagnet 

 circuit may not be affected. The date dial may, however, if desired, 

 be set independently, using the binding nut just above the large dial 

 ring for releasing and clamping. If only a small motion of the date 

 dial is necessary, it is generally preferable to set it by the operating 

 crank. The year index should be set to indicate the kind oi year. 



In the usual operation of the machine a ratchet prevents the 

 operating crank from being turned backwards, but this ratchet may 

 be released when desired by pressing on a button in the side of the 

 miachine just above the crank. After the face of the machine has 

 been thus set to register the beginning of the predictions the three 

 m.ain vertical shafts of the two component frames should be clamped 

 to prevent them from turning. 



To set the height amflitudes. — ^All the component cranks on the left 

 or height side of the machine are first turned, by means of the releas- 

 able gears on the main vertical shafts, to a vertical position, the 

 cranks of the upper range of components pointing downward and 

 those in the lower range upward, in which position all angles will read 

 180°. For the long-period components the cranks can be more 

 quickly brought to the vertical position by drawing out small knobs 

 on the time side of the machine, thus disconnecting the gearing. 

 The cranks are then turned by hand to the desired position and the 

 knobs pushed back into place. The amplitudes may now be set 

 according to the scales attached to the sides of the machine. The 

 crank pin is undamped by a small milled head wrench and is then 

 moved along its groove until the index at the scale registers the 

 amplitude setting given in Form 445, when it is clamped in this 

 position. If no amplitude is given for any component, the corre- 

 sponding crank must be set at zero. 



To set time amplitudes. — The process is similar to that for the 

 height amplitudes, the cranks on the time side of the machine being 

 first turned to a vertical position with all angles reading 90°. The 

 cranks are to be set with the same amplitudes as were used for the 

 height side, the modified scales automatically taking account of the 

 true differences in the amplitudes. For the components Sa and Ssa 

 the amplitudes are set on the height side only. 



