68 



Ocean Magnetic Observations, 1905-16 



Sea dip-circle 169. — Sea dip-circle 169 was originally of the Lloyd-Creak pattern and 

 was used throughout Cruise I as received from the maker, without modification. Deflec- 

 tion observations could be made only at one distance, and even these became impossible 

 for the greater part of Cruise I (see p. 19). After the modification of the instrument 

 (see p. 21), it was used throughout Cruise III. The adopted formulae resulting from least- 

 square adjustments of all available data for corrections to observed inclinations are given 

 in Table 13. 



Table 13. — Inclination Corrections for Sea Dip-Circle 169. 



'Corrections actually applied in original reductions of sea observations were — 1'7 for needle 1, and —3.8 for 

 needle 2; the mean negative correction for the two needles, actually applied, was, accordingly, 0'7 different from that 

 finally adopted. Since, however, the deviation-correction term Ax was determined from comparisons of harbor-swing values 

 of / with shore observations obtained with other dip circles, besides with 169, the final correction to originally adopted 

 values of /, corrected for ship's deviations, would be only of the order +0!3 — a neghgible quantity. The originally computed 

 values of / and H are, therefore, accepted. 



'Deflection method could not be used on Cruise I between Sept. 14, and Nov. 17, 1905, the deflection- 

 distance being too short. When method could be used, the observations during Cruise I were generally made with needle 

 3 both direct and reversed; corrections, when necessary, to reduce / by ZD or by 3/J to mean of 3/3 and 3/J were taken from 

 preceding and following observations for which the needle 3 was used in both positions. 



During Cruise I inclinations on course were generally observed with both polarities 

 of needle, and inclinations during swings were usually observed with opposite polarities 

 during the two swmgs; for example, if on the port-helm swing the polarity of dip needle 

 used was A, then, on the starboard-helm swing the polarity of same needle was B. Half 

 polarity-differences, when necessary, were supplied with the aid of preceding and following 

 observations with both polarities; the quantities adopted for Ci-uise III, from graphs based 

 on all available data, are given in Table 14. 



