Reductions to Standard Instruments 



17 



DIP CORRECTIONS. 



In the regular dip, or inclination, observations, the polarity of needles is 

 invariably reversed and hence the so-called balance error due to eccentric position 

 of center of gravity of the needle is eliminated. There remains, however, the error 

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

 field. Hence the determinations of needle-corrections at a base-station, however 

 carefully executed, may not necessarily apply to a region far remote, where the dip 

 and intensity are considerably different. In determining the corrections, use has 

 been made not only of the comparisons at Washington, but also of all available 

 comparisons obtained in the field and at observatories. Where the reliable com- 

 parisons have been sufficiently numerous, a tentative relation of the following form 

 has been established, F being the total intensity, / the inchnation, and x, y, z, 

 3 coefficients to be determined: 



FAl = x-{-z cos I-\-y sin / 



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 working back from these differences to the base- 

 station corrections. 



The dip corrections adopted for the various instruments used in the observa- 

 tions contained in this volume are given in Table 3; these corrections are to be 

 applied algebraically, regarding dip, north end down, as positive, and south end 

 of needle down, as negative. 



Table 3 also gives the corrections for the compass attachments of the dip 

 circles; these corrections are to be applied algebraically to observed results, regard- 

 ing east declination as positive and west declination as negative. 



Table 3. Dip Corrections on Adopted C. I. W. Standard for the Period 1911-1913. 



'For explanation of types, see p. 12. 



