Description of Sea Deflector 191 



Descriptions of the Various Sea Deflectors. 



Briefly described, the sea deflector is an application of the sine-deflection method for 

 the determination at sea of horizontal intensity. The deflecting magnet, instead of being 

 mounted in the same horizontal plane with the deflected magnet and off to one side, for 

 example, to the east or to the west, as in most forms of land magnetometers, is mounted 

 vertically above the center of the deflected magnet-system, or compass-card. The mechan- 

 ical details are such that, when the sighting device is set upon the north or south point 

 of the compass card m equihbrium, the deflecting magnet is at right angles to the north-and- 

 south diameter of the compass (assumed to define the magnetic axis of the card), and 

 hence the principle of the simple sine-deflection method is secured. The method of 

 observation is such that double deflection-angles are observed. Designating the single 

 deflection-angle by u, the value of horizontal intensity, H, is obtained by means of the 



formula 



„ _ mC 



sin u 



where m is the magnetic moment of the deflecting magnet at the time of the observation, and 

 C is a function of the deflecting distance and its changes with temperature and of the 

 induction and distribution coefficients. In practice the value of mC is determined at every 

 port visited, simultaneous observations being made ashore with the sea deflector and the 

 standardized land-magnetometer belonging to the ocean outfit. Two deflecting distances 

 are used, as also two different magnets, to eUminate, as far as possible, uncertainties in the 

 values of mC. The shore observations also supply the data necessary for the determination 

 of the temperature coefficient. From a series of such port observations, the results being 

 reduced to some standard temperature, as for example, 20° centigrade, the change in the 

 value of mC with time and magnetic field are determined. (See pp. 236-241.) 



The sea deflectors as used have been intended primarily for the detennination of the 

 horizontal intensity, but to provide desired checks on the dechnation results with the 

 standard compass they have been equipped with azimuth devices so as to permit making 

 declination observations also. In the original instruments used on the Galilee, the Ritchie 

 or the Negus standard azimuth-attachments were used for this purpose, but in the later 

 types, constructed by the Department, simpler and more readily controllable azimuth 

 devices were introduced. 



The liquid type of compass has been used for all the deflectors, the liquid acting as a 

 damping device and thus rendering it possible to make settings with great rapidity. The 

 type of deflecting magnet now used, and selected after numerous experiments with various 

 shapes and styles, is of cylindrical form, similar to those used by the Department in some 

 of its magnetometers. 



The mstruments thus far constructed may be divided uito two types, viz: A, the type 

 in which the deflecting-magnet supports are carried on a frame-work rotating on the com- 

 pass bowl and deflection angles are read du-ectly on the card graduation ; B, the type in which 

 the deflecting-magnet supports form a permanent attachment to the bowl, the bowl itself 

 being rotated when settings are made and the angles being read by vernier on a graduation 

 made on the edge of the bowl. To date five of these instruments have been constructed, 

 designated, respectively, Nos. 1, 2, 3, 4, and 5. 



Sea deflectors 1 and ^.— These were of the type .4 and in general along the same hues 

 of construction. For descriptions and illustrations, see Galilee work, pages 24-26, and 

 Plate 5. This type has been superseded by No. 3. 



Sea deflector S.— This instrument (PI. 12, Fig. 1), used on the Carnegie during the 

 whole of her first cruise, 1909-1910, and on her second cruise, 1910-1911, as far as Cape 

 Town, South Africa, is of the type B; it was designed and constructed by the Department, 



