IV. COMPASSES. 649 



An important objection had weighed with the Compass Department of the 

 British Admiralty against the use of quadrantal correctors in the navy. It 

 was, that they would obstruct the taking of bearings of celestial or terrestrial 

 objects for the purpose of correcting the compass or of terrestrial objects for 

 the navigational use of it. On this account the mirror azimuth instrument 

 now exhibited was designed. It not only does away with that objection to 

 the application of quadrantal correctors, but it is much more convenient for 

 ordinary use at sea than the prismatic arrangement hitherto in use. It faci- 

 litates very much the taking of star azimuths by throwing (as in the camera 

 lucidd) an image of the star upon the divided circle of the compass card, 

 illuminated by the ordinary binnacle lamp), or more properly speaking, on a 

 virtual image of this scale at an infinite distance as seen through a convex 

 lens. It is easy when there is not much motion in the ship to read the posi- 

 tions of the star accurately to a small fraction of the white space between two 

 dark degree divisions on which its image is seen. The focal length of the 

 convex lens is a little greater than the radius of the circle, and thus for 

 objects on the horizon or at any altitude not exceeding 30, no farther adjust- 

 ment of the azimuth appliance than just to bring the object fairly into the 

 field of view is necessary. 



The compass consists of a light aluminium boss, with a central sapphire cap 

 (by which the compass is supported on an iridium point), and a rim of 

 aluminium of from 4 to 10 inches diameter, according to the size of the 

 compass. There is an even number of holes in the rim, and half that number 

 in the circumference of the boss. The rim and boss are connected together 

 by means of fine silk threads forming, as it were, 32 spokes, and the compass- 

 card is partly supported by these threads and partly by the rim. Two or four 

 small magnets having their corresponding ends tied together by silk threads 

 of equal lengths, so that the magnets may be as nearly parallel as possible, 

 are attached to the rim by means, of four silk threads. 



The compass thus obtained, being extremely light, and having a large 

 radius of gyration, has very small frictional error, with small enough mag- 

 netic movement to give a very long period of free vibration. The smallest 

 compass (I.) has just about the same period of free oscillation as the Admi- 

 ralty standard compass, and the same quality of steadiness at sea, while the 

 larger sizes have considerably longer periods of from 28 to 43 seconds, and 

 therefore much greater steadiness at sea. 



3145aa. Marine Equatorial for correcting Compass by 

 the Sun. 



Sir William Thomson, Professor of Natural Philosophy r , 



The University, Glasgow. 



A circle corresponding to the earth's equator is set upon gimbals with 

 adjustment, by which the inclination of the circle to the vertical is made 

 equal to the latitude of the place. A lens is mounted in a doubly-pivotted 

 frame, which keeps its centre on the centre of the equatorial circle while 

 allowing the lens to turn round its own diameters through this point. 



3151. Azimuth Compass, by Brauer. 



T. Brauer, St. Petersburg. 



