254 



Ocean Magnetic Observations, 1905-16 

 Magnetometer Corrections. 



The corrections of each magnetometer on the adopted standard (see p. 232), were 

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

 whenever possible, by means of comparisons with other magnetometers. The accuracy 

 of the mean corrections for the land instruments is usually about 0(2 in declination, and 

 about O.OOOIH in horizontal intensity. The tabulated corrections are to be applied alge- 

 braically, east declination being reckoned as positive and west declination as negative; 

 horizontal intensity is always taken as positive. 



The tabulated ^-corrections are those actually applied in the final reductions of the 

 obser\'ations, except for magnetometers 5, 14, 19, and 25, for which the values as given 

 in Table 63 are the equivalent corrections on the basis of the finally adopted distribution 

 coefficients given in Table 62, instead of the distribution coefficients first adopted and used 

 in the original computations and revisions. 



Table ^tZ.—MagneUnmler Corrections on Adopted C. I. W. Standards for the Period 1909-1914. 



Dip-Circle Corrections. 



In the regular incUnation-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 gravity of the needle is eliminated. 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 observatories are utifized to determine for each needle an equation expressmg the 

 variation of the correction, M, with total intensity, F, and mclination, /, of the general 

 form (see Volume I, p. 45, Volume II, p. 17, and this volume, p. 250) : 



FAI = X + zcos I + y sinl 

 In the cases, however, where only a few reliable comparisons are available, and particularly 

 in the tropics, where, because of the development of rust, a rapid deterioration of the dip 

 needles is encountered, it has been necessary to depend for the corrections on a critical 

 study of the differences exhibited by the needles among themselves, and then to work back 

 from these differences to the base-station corrections. 



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

 instrument; they are to be apphed algebraically, mcUnation of the north-seeking end of 

 the needle below the horizon being regarded as positive, and vice versa. In case of the 

 shore stations of Cruise I, for which values obtained with the sea dip-circles are utilized 

 in the Table of Results for shore stations, the corrections given in Table 60 and on 

 pages 255 256 were appUed. The declination corrections adopted for the dip-circle compass 



