76 



Ocean Magnetic Observations, 1905-16 



Magnetometer Corrections. 



The corrections of each magnetometer on the adopted standard (see p. 77) were deter- 

 mined at Washington, before and after use of the instrument in the field, and also, whenever 

 possible, in the field by means of comparisons wdth other magnetometers. The accuracy of 

 the mean corrections for the land instruments is usually about ' 2 in declination and about 

 0.0001.ff in horizontal intensity. The tabulated corrections are to be appUed algebraically, 

 east declination being reckoned as positive and west dechnation as negative; horizontal 

 intensity is always taken as positive. 



Table 22. — Magnetometer Corrections on Adopted C. I. W. Standards for the Period 1905-1908. 



Land Dip-Circle Corrections. 



In the regular inclination observations at shore stations the polarity of the needles is 

 invariably reversed, and, hence, the so-called balance-error caused by any eccentricity of the 

 center of gra\'ity of the needle is eUminated. There remains, however, the error caused by 

 any irregularity of the figure of the pivot, and this will vary, in general, with the magnetic 

 field. The correction-data from all comparisons at Washington, in the field, and at obser- 

 vatories are utihzed to determine for each needle a formula expressing the variation of the 

 correction, A7,with total intensity, F, and incUnation, 7, of the following form (see Volume I, 

 p. 45; Volume II, p. 17, and the present Volume, pp. 71-72) 



FAI = x + z cos I + y sin I 



When only a few reliable comparisons are available, or when there has been rapid 

 deterioration of the needle-pivot, caused, e. g., by slight rusting, or when the circle is used in 

 a limited region, mean values of A/ are adopted for all values of F and /. 



The adopted dip-corrections for the land instruments are given separately for each 

 instrument; they are appUed algebraically, regarding inclination below the horizon of the 

 north-seeking end of the needle as positive, and vice versa. For shore values obtained with 

 sea dip-circles, the corrections given in Tables 10, 13, 16, on pages 66, 68, 69, were applied. 

 The declination corrections adopted for the dip-circle compass-attachments follow the 

 inclination corrections; the dechnation corrections adopted for the compass-attachments of 

 the sea dip-circles are given on page 77. 



Land dip-circle 171. — Circle 171, manufactured by Dover, was used during Cruises I, 

 II (to May 1906), and III (from March to May 1907). The adopted incUnation-corrections 

 for the mean observed values by needle 1 and 2 for all values of I between 60° N and 60° S is 

 O'O, and for all values of I between 60° N and 80° N is -1- O! 5; the adopted correction for the 

 mean of observed values by needle 5 and 6 of 172 for all values of 7 is -|- I'O. 



The adopted correction for observed declinations by the compass attachment is -f- 2 ' 5. 



