CHAP. I.] 



THE COMPASS. 



521 



magnetized needle. This angle is the magnetic declination of the 

 place at the moment of observation. 



The same method of observation is employed in measuring the 

 declination by means of Gambey's compass (Fig. 335), only this 

 instrument enables us to obtain the element in question with a still 

 greater precision. The needle here is a magnetized bar A B, whose 

 ends are provided with two rings with cross wires which serve to fix 

 the position. This bar supported in its centre by a stirrup is sus- 

 pended by a fine bundle of silk threads without torsion to a movable 

 windlass. Under the influence of the earth's magnetism it takes up 

 after a few oscillations a fixed 

 direction, which is that of the 

 magnetic meridian of the place 

 at the moment of observation. 

 The whole question consists in 

 determining with all the preci- 

 sion possible the angle which the 

 magnetized bar then makes with 

 the geographical meridian of the 

 place. The frame which supports 

 the stirrup carries at the same 

 time a telescope L, which fulfils 

 the same office as that of the 

 compass described above. The \ 

 frame which supports it, . and 

 which supports also the suspend- 

 ing thread and the bar, turns on 

 the plane of the divided edge c c, 



provided with verniers by which to read the divisions corresponding, 

 first to the position of the telescope, and consequently to that of the 

 vertical plane of the star observed, and then to the position of the 

 vertical plane containing the axis of the magnetized bar. In order to 

 avoid- the influence of the motions of the air, the silk thread is 

 inclosed in a case with glass sides, and another case, M M, incloses 

 the bar, whose extremities are then observed through the openings o o. 



The declination compass is of the greatest use to navigators, in 

 furnishing them with one of the elements necessary for determining 

 the route of the ship, that is, the angle which the vessel's course 



FIG. a34. Declination L-OIII|> 



