242 Ocean Magnetic Observations, 1905-16 



Inclination Observations. 



Sea dip-circle. Specimen observations and computations for the determination of 

 inclination, 7, on board ship with the sea dip-cu-cle, are fully shown on pages 219-222. Incli- 

 nation corrections for each needle were determined at Washington, and at each shore 

 station, by comparisons between the sea dip-circles and standardized land dip-circles or 

 standardized earth-inductors. During the Carnegie cruises mclinations were ahnost 

 always observed with both polarities of the regular dip-needles. When that was not done, 

 polarity corrections were derived from preceding and followmg observations, made with 



both polarities. , , . , ., ^ 



The deterioration of the dip-needle pivots, used with the circles on the Carnegie, was 

 more rapid than on the Galilee. It is probable that the greater trouble experienced on that 

 account was caused chiefly by the gases and waste products unavoidably present in the 

 engine and gas-producer rooms, which are quite close to the instrument store-room, sup- 

 plemented by the large temperature changes during the operation of the gas-producer 

 plant and gas engine. This rapid deterioration of pivots caused both progressive and 

 erratic time-changes in the incUnation corrections, in addition to those changes which 

 depend upon magnetic field and upon pivot-section irregularities. It was thus not possible, 

 in the Carnegie work, to rely wholly on least-square adjustments of the available data in 

 accordance with the general formula 



FA7 = x-f-zcosJ-f ysin/ 



which was used for practically all of the Galilee work. The adopted corrections, there- 

 fore, for Cruises I to IV, except for Cruise I, are based upon a combination of (1) adjusted 

 formulae corresponding to the above equation, and (2) a linear time-change between the 

 corrections determined at successive shore stations. 



Specimen observations and reductions for determination of inclination corrections are 

 given on pages 243-246. These specimens are typical of the determinations made for each 

 needle at a shore station. The order of observation followed is such that the mean times of 

 the needles in a set of determinations will be practically the same. The order, for example, 

 with a circle provided with dip needles 1 and 2, and intensity-needle pair 3 and 4, would be: 

 (1) inclination observations with polarity A north for needle 1, (2) corresponding observa- 

 tions with needle 2, (3) loaded-dip observations with needle 4, (4) deflected-dip observa- 

 tions with suspended needle 3 " f ace dii-ect " at short deflection-distance, (5) corresponding 

 observations with needle 3 at long deflection-distance, (6) deflected-dip observations 

 with suspended needle 3 "face reversed" at long deflection-distance, (7) corresponding 

 observations with needle 3 at short deflection-distance, (8) loaded-dip observations with 

 needle 4, (9) inclination observations with polarity B north with needle 2, and finally (10) 

 correspondmg observations with needle 1 . For a typical compilation, showing the observed 

 and adjusted values of inclination corrections for an entire cruise, see Table 19, page 72. 



Marine earth-inductor.— The satisfactory perfonnances of earth inductors of various 

 makes and designs, as evidenced by the extensive comparison work of the Department of 

 Terrestrial Magnetism, indicated that the inclination correction for a well-made inductor 

 is practically the same for all magnetic fields. In view of the difficulties experienced, 

 because of the changes with magnetic field in the needle inclination-corrections of the 

 sea dip-circles, and particularly because of the more or less erratic changes arising from 

 unavoidable needle-pivot deterioration, the desirabiUty of adapting the earth inductor 

 for use on board ship was early appreciated. Therefore, after an extended theoretical 

 study of the conditions involved,^ the design and construction of an earth inductor, with 

 appurtenances suitable for observation at sea, was undert aken by the Department of 



'C/. Dorsey, N. E. The Theory of the Earth Inductor as an Inclinometer. Terr. Mag., vol. 18, pp. 1-38. 



