IV. COMPASSES. 779 



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. 



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 on a doubly-pivoted 

 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. 



Part of the arrangement for effecting this consists of an outer frame, which 

 has a motion round an axis perpendicular to the equatorial circle. This 

 outer frame carries a portion of a spherical surface arranged as a screen 

 to receive the sun's image during an hour. This screen is marked with 

 declination circles, and these declination circles are divided by portions of 

 meridional circles into spaces corresponding to five minutes of time. When 

 the instrument is set with the axis of the equatorial circle truly parallel to 

 the earth's axis, the sun travels along the declinational circle corresponding 

 to his declination at the time. The frame carrying the screen is turned into 

 such a position that the two extreme meridional circles of the screen agree 

 with numbers on the equatorial circle, corresponding to the integral hours of 

 apparent time before and after the time of observation. Then the instrument 

 is turned in azimuth till the sun's image falls on the proper declination circle, 

 ar"l the point of it corresponding to the apparent time- at the moment of 

 observation. If there are no instrumental errors and if the adjustment for 

 latitude is perfectly correct, then it is sufficient to set the instrument in 

 azimuth till the sun's image falls with absolute accuracy on the proper decli- 

 natioiial circle. Besides showing the true North, the instrument then shows 

 as a sun-dial the apparent time, or if the apparent time is known with perfect 

 accuracy, and the instrument is turned in azimuth till the sun's image is 

 brought to mark correctly the apparent time, then the instrument shows the 

 sun's declination. In practice both indications are looked to. When the 

 instrument is used in equatorial, regions, however, it is solely or almost 

 solely by the sun's declination that it is set so as to give the true North. 



In high latitudes, North or South, it is almost wholly from independent 

 knowledge of the apparent time that the instrument is set. Generally the 

 directions to the navigator using it are to set it to agree as well as he can 

 both with the apparent time and with the sun's declination. 



The instrument now exhibited has been tested at sea on board the Cunard 

 steamers " Russia " and " Scythia " in voyages from Liverpool to New York 

 and back, and has been found to act well for the purpose for which it was 

 designed. It is, however, defective, and inconvenient in several details, which 

 will be improved in instruments of the kind to be made in future. 



