REDUCTION FORMULAE AND DETERMINATION OF CONSTANTS. 



REDUCTIONS TO STANDARD INSTRUMENTS. 



The extensive intercomparisons of magnetic instruments at Washington, in 

 the field, and at magnetic observatories in all parts of the Earth, carried out by 

 the Department of Terrestrial Magnetism, have made it possible to refer all its 

 data to international magnetic standards within an error, in general, on the order 

 of the observational error (see Volume II, pp. 211-278). Since the adopted con- 

 stants for the sea instruments, as explained in the subsequent paragraphs, were 

 made to depend upon the standardization data at shore stations (see pp. 296-309), 

 the results derived from the magnetic observations on board the Carnegie are on 

 the basis of the adopted magnetic standards. 



Magnetic Standards Adopted. 

 The magnetic standards adopted for reduction to a common basis of the 

 results contained in the present volume are the so-called "C. I. W. Standards," as 

 defined in Volume I (p. 42) and II (p. 16). These "C. I. W. Standards" are: 

 In decUnation, C. I. W. magnetometer 3 without correction; in horizontal intensity, 

 C. I. W. magnetometer 3 with a correction of -1-0.00015// appUed to observed 

 values of the horizontal intensity, H, computed by the constants given for magnet- 

 ometer 3 in Table 62, page 253; in inclination, earth inductor 48 with a correction 

 of — 0'.5 apphed to observed values of inclination. A detailed discussion of the 

 relations between the "C. I. W. Standards" and possible "International Magnetic 

 Standards" is given in Volume II (pp. 270-278). It is shown there that the cor- 

 rections of the originally selected standards are so small as to be negUgible here. 

 Accordingly, the values of the magnetic elements, given in the Tables of Results on 

 pages 261-309, may he regarded as based on "International Magnetic Standards." 

 CONSTANTS AND CORRECTIONS FOR SEA INSTRUMENTS. 



The instrumental constants and corrections on standards (above) of the sea instru- 

 ments used in the Carnegie work were determined at Washington and at the various ports 

 visited by comparisons with standardized land-instruments. The method adopted in the 

 comparisons was generally that of sunultaneous observations. In order to refer values of 

 the magnetic elements at one observing station to any of the others, station differences were 

 carefully determined at each port from observations with the land instruments, following 

 the methods described in Volume I (pp. 19, 20). 



Declination Observations. 



Marine collimating-compass. — The marine coUimating-compass, designed and con- 

 structed by the Department of Terrestrial Magnetism, has been used as the standard 

 declination-instrument for the Carnegie work. Description of the compass, its theory, and 

 explanation of its use, will be found on pages 177-190. Specimen observations and computa- 

 tions for a sea station are given on pages 213-215. The introduction of the collimating 

 principle has facilitated the control of instrumental constants, so that in the field only 

 two have to be determined for each scale, viz, (1) the magnetic-axis and index error, 

 Ac, and (2) the elevation, m. The constants can be determined at shore stations with an 

 accuracy much greater than that for declination determinations at sea, and are, further- 



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