Reductions to Standard Instruments 



61 



for the method of sighting used, viz, prism or ahdade, may be represented by the formula 

 (see pp. 134-140) 



A„ or Aac = x + y tan h + z tan h tan ^ + w tan h see x 



in which x is the index correction as above determined, h is the altitude of the celestial body 

 observed, and y, z, and iv are coefficients which may be determined by least-square adjust- 

 ment of a number of observations at different altitudes. The data for the establishment 

 of such a formula for the azimuth circle of each compass were secured at the shore stations 

 from series of observations made on the Sun with this instrument to determine the magnetic 

 decUnation. The absolute values of the magnetic decUnation were determined from obser- 

 vations with the standardized magnetometer. Depending upon the sighting device (prism 

 or alidade) used, the total correction then, is either e + Apc or e + Aac- 



Each of the terms making up the total correction to observed cai'd-reading, viz, « 

 and A^ or A„, is given separately in this section for each compass and azimuth circle. 

 The signs attached are in the sense of continuous graduation from the south point as 0° through 

 360° in a clockwise direction; therefore all card readings in the southwest and northeast 

 quadrants— that is, all readings from S to S 90° W (or W) and from N to N 90° E (or E)— 

 must be numerically increased when the sign given for e, Ape, or Aac is plus (+), while all 

 card readings in the other two quadrants must be nmnerically decreased when the sign 

 given for e, A^, or Aac is plus (+), and vice versa. 



Standard Ritchie compass 29971 xoith azimuth circle 387-1 II (RIA). — The prism method 

 alone was used with this equipment, which was on board during Cruise I. The corrections 

 were included and applied in the ship's deviations, comparisons having been made directly 

 between declinations observed on board and corresponding values observed on shore with 

 the standardized magnetometer. 



Standard Ritchie compass 29971 with azimuth circle 41 8-1 1 1 (RIB). — The adopted 

 periodic corrections to observed card-readings of compass RIB, used on Cruise II, are as 

 foUows: 



Table 7. — Periodic Corrections to Card Readings of Compass RIB. 



The correction. Ape , to observed card-readings by the prism method was found to be 

 independent of the Sun's altitude, but there were two marked changes in its value; the 

 values adopted are 



Ap, = -f 1?10 from February 14 to April 25, 1906 



Ape = -0?62 from May 10 to May 18, 1906 



Ape = +0?18 from May 20 to October, 1906 



The adopted correction, Aac, to observed card-readings by the ahdade method, deduced 

 from a least-square adjustment of all available data, varies with the Sun's altitude, h, and 

 is given by the formula 



A„=-t-0?13+0?00 tan;i-l?06 tan;itan2+0?30 tan/iseCg 



