Description of Sea Deflector 193 



The arrangements for declination work with No. 4 are in general similar to those of 

 No. 3. A long sUt is provided for bright-Une work, this mounting being set at an angle 

 of about 55°; a sighting wire is also added for use in mark readings and for decimation work 

 by the shadow method. A universal level is mounted at the center of the glass surface of 

 the bowl and supported by means of arms attached at the side. In case the level is broken, 

 the arrangements are such that it may be quickly removed and another put in its place. 

 When the Sun is sighted upon for determining the magnetic decUnation, a sharp, bright 

 line is thrown through the slit of the inclined vane upon the surface of the glass and a 

 setting is made by shifting the bowl so that this bright line coincides with a central line 

 etched on the glass, the card reading being made simultaneously. When the Sun is too 

 low for the bright-line method, direct observations on the Sun may be made by sighting 

 through the incUned vane-sight, using shade glasses as necessary. The shadow of the ware 

 of the second vane is also sometimes used. In order to faciUtate using either the bright- 

 line method or shadow method at lower altitudes than was possible with No. 3, two ground 

 glasses have been mounted in the frame carrying the level on which Unes are etched and 

 suitably adjusted, so that, when the instrument is level, they are in the vertical plane 

 through the sights and the etched line on the surface of the glass. 



Two cover plates, not shown in Plate 12, are provided for use with the instrument when 

 required. They are mounted on either side of the level support, their purpose being to cut 

 out any troublesome reflections from the glass surface; it was not generally necessary to 

 use them, however. In order that the reflection of the opening of the wire sight may be 

 cut off from the glass surface and not interfere with the direct readmg of the card, a metal 

 slide was added; there is also attached to the level-supporting frame a shield in such a 

 position that the image of the sUt of the bright-Une vane will not interfere with the reading 

 of the card when using the bright-line method. 



For measuring angles between prominent objects when entering a harbor, for example, 

 there is a sighting rmg for which a suitable bearing has been provided in the upper part 

 of the bowl. Figure 3 of Plate 12 shows this sighting device as mounted for use. Two 

 opposite sides of this ring are partially cut off in order that it may be easily mounted or 

 dismounted, and a reading prism is attache'd by means of which the card may be read simul- 

 taneously with the sighting of the mark. The two vanes of this device are inclined in 

 order that there may be no interference with other parts of the instrument. 



The double-defiection distances are, respectively, 344, 364, 384, and 402 mm., the 

 bearings for the deflection magnet being such that the corresponding deflection distances, 

 above and below, will be the same, as nearly as possible. Owing to the lack of symmetry 

 of the card magnets and impossibiUty of alining them absolutely under the condition of 

 soldering necessary in construction, the actual, equivalent vertical eccentricity of the card 

 magnets, with reference to the deflection distances, appears to be of the order of 3 mm. 



Sea deflector 5.— This latest form of sea deflector was also constructed in the Depart- 

 ment's instrument shop; it was supplied to the Carnegie at Port Lyttelton in April 1916. 

 While similar in design and construction to No. 4, it contains the following improvements, 

 suggested in part by the observers: 



(a) The compass magnets are soUd bars of tungsten steel, carefully magnetized by 

 the methods used for the magnets of the Department's land magnetometers; (6) the 

 matter of centering and mounting of the compass magnets is executed with special regard 

 to accurate adjustment and balancing of magnet-system; (c) the lubber-lines are in black 

 on the poUshed-silver reflecting surface; (d) the surface of the graduated German-silver 

 circle is in the same plane as the pivot point (in No. 4 this plane is 1.7 mm. above the pivot 

 point) ; (e) the filling cap is on the side instead of below, as is the case in No. 4, thus making 

 it possible, in emergency, to fill the bowl without removing either deflecting-magnet 

 standard; (/) two sliding shades have been provided for the sightmg vane L2; (g) the 



