520 THE APPLICATIONS OF PHYSICAL FORCES. [BOOKV. 



the north-seeking pole was towards the shoulder. Afterwards, in the 

 second century, the Chinese compass had another arrangement which, 

 through the Arabs, was communicated to European navigators at the 

 time of the first crusades. This was a magnetic needle on a floating 

 support. It was not till towards the first half of the fourteenth 

 century that this instrument, so useful in navigation, so precious in 

 these days for the physical study of the globe, received a new improve- 

 ment, and the magnetic needle was supported on a pivot. 



Without stopping too long on the history of the compass and its 

 application to navigation and the arts and sciences, we will rapidly 

 pass in review the laws of magnetic orientation, and describe the 

 apparatus as they are now employed for various purposes. 



A magnetized needle freely suspended by its centre of gravity, and 

 free to oscillate in every direction about that point, takes, when it is 

 in equilibrium, a position which makes an angle both with the 

 meridian and with the horizon of the place. The first is called the 

 angle of declination, or simply the magnetic declination ; the second is 

 the 'magnetic inclination, whence there are two kinds of compasses, 

 according as it is intended to determine the one or the other of these 

 physical elements. 



We shall first deal with the Declination Compass. 



When a scientific determination is required, the declination com- 

 pass is constructed as in Fig. 334. The magnetized needle is supported 

 on an agate pivot, and inclosed in a cylindrical case M which carries 

 on two metallic mounts a telescope L L', provided with cross wires at 

 its focus and movable itself about an axis a a', parallel to the plane 

 of the instrument's edge. All this system can itself turn horizontally 

 upon this plane which is bounded by a divided circle r Q. 



To measure the declination the compass is placed on a nearly 

 horizontal surface, and its perfect horizontality is secured by observing 

 the spirit-level b b'. This done, the telescope is turned to a known 

 star, and from the time of the observation, the angle may be calculated 

 which the vertical plane containing the star and the telescope makes 

 with the meridian, which is called the star's azimuth. From this the 

 direction of the meridian is fixed on the edge i Q. The inner rim is 

 then turned on the circle P Q by a quantity equal to that angle ; the 

 line of vision NS, 180, is then on the meridian, and it only 

 remains to read upon the circle M the angle which it makes with the 



