MAGNETISM. 



69 



mit of its being turned round horizon- 

 tally, and placed in all azimuths. To 

 the upper extremities of this semicircle, 

 a brass circle, CD, is fixed by two 



pivots GG, constituting; a horizontal 

 axis of motion ; while the inner cylin- 

 drical brass box, PQ, containing the 

 compass itself, is attached to the brass 



Fig. 63. 



circle, CD, 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 piece of glass. 

 The two sights, E and O, are fixed ver- 

 tically 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, consists of a 

 brass slip, having a narrow vertical 

 slit ; the other, O, which is turned to 

 the object, is a similar slip, having an 

 oblong aperture containing a fine 

 thread, or horse hair, passing along the 

 middle of the open space in a vertical 

 direction. Two vertical lines are also 

 marked on the inside of the box, which 

 are prolongations of the slit in the sight 

 for the eye, and of the thread in that 

 for the object. These lines are intended 



as indexes for the measurement of the 

 angular distance in azimuth of an ob- 

 ject viewed through the two sights, 

 from the place of the magnetic meri- 

 dian, as shown by that portion of the 

 graduated edge of the card, which coin- 

 cides with the line with which it is com- 

 pared. The degrees are reckoned from 

 the north point of the compass, which 

 is marked zero, all round the circle, in 

 the direction from left to right, that is 

 from north to east, and thence to south 

 and west. 



(268.) Sometimes a wire is placed be- 

 tween the two sights, stretching hori- 

 zontally from the foot of the one to that 

 of the other. This is intended as an 

 index to ascertain coincidences with the 

 degrees marked on the card, when they 

 are viewed from above : but as this is 

 a mode of using the instrument that is 

 seldom practised, this wire is usually 

 omitted, and observations made solely 

 by means of the vertical lines. 



(269.) On one side of the box con- 



