236 



Ocean Magnetic Observations, 1905-16 



Ai^ or A,c, to observed card-readings by the "bright-line method," or by the "shadow 

 method," deduced from graphical adjustments of all available data, are given in Table 55. 



Table 55. — Corrections to Observed Card-Readings of Compass D4. 



Horizontal-Intensity Observations with Sea Deflector. 



As stated on page 191 and shown on the specimen form, page 217, the horizontal 

 intensity is computed from sea-deflector observations by the formula 



rj _ mC 



in which to is the magnetic moment of the deflecting magnet, C is a constant involving 

 the deflection distance r, the distribution coefficients P and Q, the induction factor 

 ^ = mh {h being the induction coefficient for the deflecting magnet), and u the observed 

 angular deflection produced by the deflecting magnet when its axis is perpendicular to 

 that of the compass. The sea deflector is a relative instrument, and values of the so-called 

 constant, mC = H sin u, must be detennined from comparison horizontal-intensity obser- 

 vations, made at shore stations with standardized absolute instruments. 



The constant, mC, is subject to changes arising from (1) decrease in m with time, (2) 

 effects of temperature variations on 771 and r, and (3) effects of change in vertical intensity, 

 Z. In the Carnegie work aU available data for log mC, except as noted below under the 

 constants for deflector 3, were subject to least-square adjustment based on the general 

 form 



log mC = log TOC20 at To + x^T + y {z - Zf + 9 (20°- t) 



in which r is the date of observation expressed in years, ro is the selected reference date 

 At is (t — To), 5 is the factor representing the combined effect of a change in temperature 

 of 1° centigrade on m and C (on the latter because of the change in r), and t is the 

 temperature of observation; the standard temperature of reference is 20° centigrade. 

 Instead of deriving all the unknowns simultaneously it is found better to make a separate 

 determination of the temperature factor, g, selecting the observations best suited for that 

 purpose. The final results were arrived at by a process of successive approximations, in 

 the last steps of which q was treated as a constant. Specimen determinations of mC at a 

 shore station, and a table showing the observed and adjusted values of that constant for 

 deflector 4 on Cruise II, are given on pages 240 and 241. 



