192 Ocean Magnetic Observations, 1905-16 



the improvements introduced being based on the experience obtained with deflectors 1 

 and 2 on the Galilee's cruises. The first instruments had shown some inherent defects 

 caused chiefly by the fact that navigation compasses already on hand had to be used. The 

 improvements in No. 3 were along lines similar to those introduced in No. 4, differing in 

 detail as indicated in the following description of the latter instrument. 



Sea deflector 4- — Additional experience showed that it would be preferable to make the 

 observations with deflecting magnet both above and below the card, that the graduation 

 ordinarily found on even the best of compass cards was not as good as was desirable, and 

 that a continuous graduation from 0° to 360° would prove advantageous. Furthermore, 

 the varying conditions of temperature encountered in ocean work and the accompanying 

 unequal expansion of the various instrumental parts caused at times a sticking or binding 

 of the bearing surfaces, thus making it hard to secure accurate settings. To overcome 

 these difl[iculties sea deflector 4 was designed and constructed by the Department; this 

 instrument is shown in Plate 12, Figures 2-9. It will be described in detail, as it is the 

 type used on board the Carnegie since 1911. 



It differs from No. 3 in that there are now two standards or deflecting arms, so that the 

 magnet may be mounted either above or below the compass cai'd. Each standard has 

 provision for 4 corresponding deflection distances. These deflection distances are approxi- 

 mately the same as were those in No. 3, viz, 172, 182, 192, and 201 mm. The upper 

 standard, with the exception of the clamping device for holding the deflecting magnet in 

 position, is made in one piece and so designed that it also serves in a general way to protect 

 the sight vanes used in connection with the declination work. For avoiding parallax, two 

 pieces are attached to the upper deflection standard, having "V" cuts placed centrally 

 with the line of sight of the vanes. (See PI. 12, Fig. 4.) 



The card graduation for this instrument (see PI. 12, Figs. 7 and 9) is made on German 

 silver, and is continuous from zero at the magnetic north point in a clock\vise direction, as 

 seen from above, thi'ough 360 degrees, the least count being 1 degree. Every 10-degree 

 graduation is numbered, thus, 1, 2, 3, 4, etc., to 35. The 5-degree graduations are dis- 

 tinguished by somewhat greater length than single-degree graduations. The diameter of 

 the graduation of the card is 178 mm. The surface of the compass magnets and of the 

 brass flange carrying the graduated circle, as well as the graduated surface, have all been 

 silver-plated and made black, so that the graduations stand out as white lines against a 

 black background. The bottom of the inside of the bowl has also been blackened, but the 

 sides have been left with the bright silvered surface in order to provide, by reflection, 

 suitable illumination for the graduation. The north point of the card has been marked by 

 an arrow on the brass sm-face of the supporting flange and the other cardinal points have 

 been marked by means of straight lines. As in the case of No. 3, the rim of the bowl is 

 graduated into 1-degree intervals, every 10-degree graduation being numbered; two 

 verniers are provided which permit reading to 0?1 and estimation to 0?05. The diameter 

 of this graduation is 248 mm. 



The bowl, as in the case of No. 3, has a cone bearing in its inner gimbal-rmg. In 

 order, however, to effect greater ease of motion and, secondly, accuracy of setting, a ball 

 bearing is provided, the adjustment of which is such that the cone surfaces are used for 

 centering purposes, while the balls carry the weight of the instrument. The detail of this 

 bearing is shown in Plate 12, Figures 5 and 6. Directly over the knife-edge supports of 

 the bearing ring are two pinions with milled heads, which may be used in finer setting of 

 the bowl, the rack for this motion being fastened to the under side of the graduated rim. 

 A single clamp is provided for clamping the bowl in its bearing; this clamp, however, is 

 not for use in the sense of a slow motion in connection with the rack, but is merely provided 

 to hold the bowl in position when necessary, and for u.se with the device for measuring 

 angles between prominent objects when entering or leaving a harbor. 



