326 POPULAR LECTURES AND ADDRESSES. 



meter circle at the top is scarcely needed, as it is 

 easy to estimate the direct reading on the straight 

 scale to a tenth of a division, which is far more 

 than accurate enough for all practical purposes. 

 This reading with a proper constant added gives, 

 in each case, the number measuring in arbitrary 

 units the magnitude of the direct force on the 

 compass for the particular course of the ship on 

 which the observation is made. 



The adjustment by aid of the deflector is quite 

 as accurate as it can be by aid of compass marks 

 or sights of sun or stars, though on a clear day at 

 any time when the sun's altitude is less than 40, 

 or on any clear night, the adjuster will of course 

 take advantage of sights of sun or stars, whether 

 he helps himself also with the deflector or not. 



The deflector consists of two pairs of small 

 steel bar magnets attached to brass frames, jointed 

 together and -supported on a sole-plate, which is 

 placed on the glass cover of the compass-bowl 

 when the ' instrument is in use. The two frames 

 carry pivoted screw nuts, with right and left 

 handed screws. A brass shaft, with right and left 

 handed screws cut on its two halves, works in 

 these nuts, so that when it is turned in either 

 direction one of the two pairs of north poles is 

 brought nearer to, or farther from, one of the two 

 pairs of south poles, while the other two pairs of 

 north and south poles are all in the line of the 

 hinged joint between the two frames. This ar- 



