SURVEYING INSTRUMENTS. 

 t- 83. 



55 



observer to read off the number of degrees from the card, 

 which correspond with an index, or perpendicular line, 

 drawn on the inside of the box. 



Description. The semicircle A B is fixed by a screw at its 

 middle, or lowest point, to a stand at the bottom of the 

 outer box containing the whole apparatus, in such a manner 

 as to admit of its being turned round horizontally, and 

 placed in all azimuths. To the upper extremities of this 

 semicircle a brass circle c D is fixed by two pivots, constitut- 

 ing a horizontal axis of motion ; while the inner cylindrical 

 brass box p Q, containing the compass, is attached to the 

 brass circle c D by similar pivots, of which one is seen at g, 

 forming a horizontal axis at right angles to the former, and 

 both together acting as gimbals. The compass with its 

 card is balanced in the usual manner on a pointed pivot 

 rising from the centre of the bottom of the inner box, the 

 upper side of which is covered with a circular glass. The 

 two sights E and o are fixed vertically on the upper side of 

 the cylinder of this box, diametrically opposite to each other ; 

 the one, E, to which the eye is intended to be applied, con- 



