24 Ocean Magnetic Observations, 1905-16 



Description of Sea Deflector. 



For the reasons above set forth there was, accordingly, developed a deflecting arrange- 

 ment based on the sine-deflection fonnula, which implies that the deflecting magnet shall 

 be at right angles to the deflected one when the state of equihbrium has been reached. 

 The deflecting magnet was mounted vertical^ above the center of suspension of a magnet 

 system (compass card), instead of in the same horizontal plane with it and off to one side, 

 e. g., to the east or west, as is done in most forms of land magnetometers and as was also the 

 case in Neumayer's "deviations magnetometer," used on the (rauss. This new instru- 

 ment is here termed the "sea deflector"; in various forms it has been used throughout the 

 work of the Galilee and the Carnegie for determining both the magnetic declination and the 

 horizontal intensity. 



For experimental sea-deflector 1, used on the Galilee's cruises up to July 30, 1907, 

 there was utilized for base of the instrument an 8-inch Ritchie-Negus liquid compass 

 (No. 31974), the kind ordinarily employed in navigation. Next a bridge, with a disk on 

 top for carrying the deflecting magnet, was attached at right angles to the sight line or sight 

 bows of the latest form of Negus azimuth-circle, provided with the said liquid compass. 

 These sight bows consisted of two stout parallel brass wires bent into bows, somewhat over 

 a millimeter apart; they served to define the vertical sight-plane passing between them 

 and through a brass pointer, with the aid of which the compass was read, or any point of 

 the card set upon; they took the place of the telescope in the land magnetometer. (See 

 Plate 5, Fig. 1.) 



To make a setting with the deflecting magnet mounted on the disk, the azimuth circle 

 was turned, carrying the deflector and sight bows, until the brass pointer was over the 

 south end of the compass card. Then, since the magnet, by construction, was mounted 

 at right angles to the sight line or bows, and as the latter were set directly over or parallel to 

 the north-and-south diameter of the compass (assmned for the present to define the mag- 

 netic axis of the compass card), it followed that, in the position of equilibrium between 

 magnet and card, the magnetic axes of the two were at right angles to each other; thus the 

 condition of the simple sine-deflection method was secured. 



Both lubber-lines, marked on the inside of the compass bowl, were then directly read 

 on the compass card to the nearest tenth of a degree, holding the eye so as to avoid parallax. 

 In this way one of four operations required to complete a set was carried out. Let us say, 

 in operation n, the north end of the deflecting magnet was towards the east, and the setting 

 of the brass pointer, with the aid of the bows, was made on the south point of the compass 

 card; then, in b, the azimuth circle would be turned so as to make a setting on the north 

 point of the compass card, the north end of the magnet then being to the west; next, c, 

 the magnet was turned around on its support, so that north end would be east, setting, how- 

 ever, again on north point of compass ; and finally, d, azimuth circle was turned and pointer 

 set on south point of compass, north end of magnet being then west. In brief, practically 

 the same four deflection positions usual in land magnetometers could be carried out with 

 the sea apparatus. 



The difference in the lubber-line readings for operations a and b, or c and d, or b and c, 

 or a and d, gave twice the angle by which the compass card was deflected from the magnetic 

 meridian owing to the presence of the deflecting magnet above it, and the mean of the 

 two readings of any one of those pairs gave the magnetic meridian, barring errors due to 

 eccentricity of mounting and of magnetic axes. The mean deflection-angle would be free 

 from errors, due to these two causes. The magnetic-meridian reading of the card was also 

 recorded before the deflecting magnet was mounted, and again after removal. The tem- 

 perature of the magnet was read, and the time was recorded, both at the beginning and 

 ending of each set of four deflection-readings. 



