196 Ocean Magnetic Observations, 1905-16 



We have not found it advantageous on a cruise to vary the weight or its position 

 from one end of the needle to the other. According to test observations at Washington, 

 or at some port, the most suitable weight, answering as well as possible the theoretical 

 requirements for a cruise, is selected. Thereafter no change is made, except, of course, in 

 case of accident. The intensity constant is, however, controlled by shore observations 

 at every port of the cruise. 



In general, if sufficient opportunities are presented to make control observations of 

 the intensity constants at ports about once a month, and if adequate care is taken of the 

 intensity needles, it is quite possible to ehminate the more or less troublesome loaded-dip 

 observations and rely entirely on the deflection observations, if they are made with the 

 precautions taken on the Carnegie. 



MARINE EARTH-INDUCTOR FOR INCLINATION. 



One of the problems^ encountered in ocean magnetic work covering an extensive area 

 has been the satisfactory determination of the variations, with change in magnetic latitude, 

 of the incUnation-corrections on standard for dip-circle needles. This problem has been 

 the more difficult because of the mechanical impossibility (despite the most skillful and 

 careful workmanship) of producing a perfect axle. Up to the present time the corrections 

 on standard have been obtained with the aid of the observation at the ports visited, where 

 intercomparisons were secured between a standardized earth-inductor and the sea dip- 

 circles. A second problem, as yet unsolved, is the determination of possible dynamic 

 effects^ upon the direction taken by the needle during observations because of the ship's 

 motion. Since the shore stations are often widely separated, and the conditions aboard 

 ship, where the instruments and needles are in constant motion, are so very different from 

 those on shore, it became desirable that some form of instrument should be devised for the 

 purpose of securing standardizations and the required control on board ship, i. e., in transit 

 from port to port. The satisfactory performance of the earth inductors of various makes 

 and designs, as evidenced by the extensive intercomparison work of the Department of 

 Terrestrial Magnetism,^ indicated that this type of instrument, if it could be made apph- 

 cable for observation at sea, might be so utiUzed. After an extended theoretical study* of 

 all the conditions involved, the design and construction of the desired apparatus in the 

 Department's instrument shop were undertaken under J. A. Fleming's supervision. 



Description.' 



The elements of the marine earth-inductor are essentially: (a) an improved form of 

 gimbal stand that wUl maintain an average mean position of equilibrium and will permit 

 complete reversal of the gimbal rings and bearings in order to eliminate errors of level; 

 (6) a portable form of earth inductor with such means for rotating the coil as will not in 

 any way, when in use, disturb the gimbal rings, and (c) a galvanometer of sufficient sensi- 

 biUty suitable for use at sea. 



Gimbal stand. — Heretofore the gimbal stand used has been of the type manufactured by 

 Dover, of England. It consists of a suitable deck-support with a U-shaped arm at the 

 top carrying a heavy supporting ring. Two brass knife-edges, mounted diametrically 

 opposite in this ring, support a second ring, which in turn carries (at the ends of the diameter 

 at right angles to and in the same horizontal plane as that used in the first ring) two brass 



'C/. Bauer, L. A. Some of the Problems of Ocean Magnetic Work. Terr. Mag., vol. 14, pp. 164-166, 1909. 



'Preparations are under way for an experimental investigation of this problem at Washington by means of observations 

 on a platform which may be given various motions corresponding to those of a ship at sea. 



'C/. Fleming, J. A. Comparisons of Magnetic-Observatory Standards by the Carnegie Institution of Washington. Terr. 

 Mag., vol. 16, pp. 61-84 and 137-162, 1911. 



■■C/. Doraev, N. E. The Theory of the Earth Inductor as an Inclinometer. Terr. Mag., vol. IS, pp. 1-3S, 191.3. 



K'f. i. A. Fleming's article in Terr. Mag., vol. 18, pp. 39-45, 1913. 



