Reductions to Standard Instruments 



Table 3. Magnetometer Corrections on Adopted I. M. S. for the Period 1914 to 1920 Continued. 



11 



1 There is indication of a change in K during the observer's ocean trip to Peru after Washington standardizations of Aug. 

 28 to Sept. 1, 1916; the value used for field observations during January 1917 to July 1918 is controlled by comparisons 

 in January 1917 at Arequipa with C. I. W. magnetometer No. 10 and in July 1918 with standard C. I. W. magnetometer 

 No. 3 at Washington, the values being 0.00072// and 0.00090// respectively. 



INCLINATION CORRECTIONS. 



As in the past for determinations of inclination with the dip circle, the polarity of the 

 needle is invariably reversed, eliminating any so-called balance-error due to eccentric 

 position of the center of gravity of the needle. There remains, however, the error due to 

 irregularity of figure of pivot, and this will vary, in general, with the angle of inclination. 

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

 executed, may not necessarily apply to a region of different inclination. The wide field 

 experience of the Department had already indicated by 1910 that in order to obtain re- 

 liable results with a dip circle, it was necessary to observe at each station, whenever 

 possible, with 4 needles, and, furthermore, to obtain at every opportunity, control of the 

 dip-needle corrections by comparisons with other dip circles and, in particular, with earth 

 inductors. From the accumulated observational data, it was frequently possible to es- 

 tablish relations by least-square adjustments for each needle in the form 



FAl = x+z cos I+y sin / 



in which F is the total intensity, J is inclination, A/ is the needle-correction on standard, 

 and x, z, and y are coefficients obtained by the method of least squares. 



Unfortunately, even when reliable comparison-data were available, the development 

 of tiny rust-spots on the pivots in the course of field work, especially in tropical regions, 

 has made it necessary in almost every case to depend for the corrections upon a critical 

 study of observed needle-differences. The prime purpose of such a discussion has been to 

 adjust the values obtained from each of the 4 needles to the mean of all, and to determine 

 upon the allowable ranges in the inclination results, for guidance in rejection of any 

 values. The large accumulation by the Department of well-distributed inclination data 

 during 1914 to 1920 has furnished material for some interesting discussions of the effects 

 of minute pivot-defects (see pages 359 to 371 of this volume). 



