Reduction Formula and Instrumental Constants 247 



Total-Intensity Observations. 



Sea dip-circle. — Complete specimen observations and computations for total intensity, 

 F, with the sea dip-circle, and the indirect determination of inclination, /, from the deflec- 

 tion observations, are shown on pages 219 and 220. The value of the horizontal intensity, 

 H, is obtained by the formula 



H = F cos I 



As the method employed is a relative one, it is essential that no change be made in the 

 weight used with the loaded-dip needle, and that its position be not shifted from one end 

 of the needle to the other during a cruise; furthermore, the magnetism of the loaded-dip 

 and deflected needles, except for the normal changes with time, must remain unchanged. 

 The reduction formulae for the total intensity are: 



Loaded-dip observations only, F = C, cos 7' esc u 



Deflection observations only, F = Ca esc mi 



Both loaded-dip and deflection observations, F = C Vcos I' esc u esc % 



where I' is the loaded-dip angle, Ui is the deflection angle, u = I — I', C, is the loaded-dip 

 IT 



constant = — , C is the deflected-dip constant = Kim, and C is the combined constant = 



■\/KK\. The constants Ci and Ci involve the magnetic moment, m, of the loaded-dip 

 needle, and are both, therefore, subject to change with temperature and with time. Cj, 

 furthermore, involves the induction correction which is a function of F. d is affected 

 also by changes in deflection distances, due to temperature changes, as well as by any 

 changes in the distribution coefficients. Two deflection distances, designated short (»S) 

 and long (L), are provided in the modified sea dip-circle (see p. 195), and thus there are 

 two independent sets of constants. In deflection observations there are also two positions 

 of the deflected or suspended magnet, designated "direct" (D) and "reversed" {R); 

 "direct" position means that the face of the deflected needle is towards the face of the 

 vertical circle; "reversed" position means that the face of the deflected needle is towards 

 the back of the vertical circle. For all of the Carnegie work the deflection observations 

 were made in both "direct" and "reversed" positions for each determination, and, there- 

 fore, the constants to be controlled by shore observations for that work are: C,, CdOR for S, 

 and CiDR for L. Values of these intensity constants were determined at each shore station 

 and at Washington by means of comparisons between the sea dip-circles and standardized 

 land magnetometers and incUnation instruments. 



Specimen observations and reductions for the determination of the constants are given 

 on pages 248-250. The specimens are typical of the compilations made for each pair of 

 intensity needles. The order followed in the observations is such that the mean times of 

 the three determinations of constants will be practically the same. The order is as 

 follows: (1) loaded-dip observations, set I; (2) deflected-dip observations for "direct" 

 position and short distance; (3) deflected-dip observations for "direct" position and long 

 distance; (4) deflected-dip observations for "reversed" position and long distance; (5) 

 deflected-dip observations for "reversed" position and short distance; and finally (6) 

 loaded-dip observations, set II. 



Because of the development of microscopic rust-pits on the needle pivots there are 

 erratic changes in the intensity constants. It was, therefore, necessary to depend entirely 

 upon graphical adjustments, or upon linear interpolations with time between shore-station 

 values. The method adopted for each cruise is given with the summary of constants 

 (pp. 250-252). 



