SURVEYING INSTRUMENTS. 5*1 



In the preceding figu- e, A represents the compass box, and 

 B the card, which, being attached to the magnetic needle, 

 moves as it moves, round the agate centre, a, on which it 

 is suspended. The circumference of the card is usually 

 divided to 30' of a degree ; c is a prism, which the 

 observer looks through in observing with the instrument. 

 The perpendicular thread of the sight-vane, E, and the 

 divisions on the card appear together on looking through th( 

 prism, and the division with which the thread coincides 

 when the needle is at rest, is the magnetic azimuth of what- 

 ever object the thread may bisect. The prism is mounted 

 with a hinge joint, D, by which it can be turned over to 

 the side of the compass box, that being its position when 

 put into the case. The sight-vane has a fine thread stretched 

 along its opening, in the direction of its length, which is 

 brought to bisect any object, by turning the box round 

 horizontally ; the vane also turns upon a hinge joint, and 

 can be laid flat upon the box, for the convenienceof car- 

 riage. F is a mirror, made to slide on or off the sight-vane, 

 E ; and it may be reversed at pleasure, that is, turned face 

 downwards ; it can also be inclined at any angle, by means 

 of its joint, d ; and it will remain stationary on any part of 

 the vane, by the friction of its slides. Its use is to reflect 

 the image o'f an object to the eye of the observer when the 

 object is much above or below the horizontal plane. When 

 the instrument is employed in observing the azimuth of the 

 sun, a dark glass must be interposed; and the colored 

 glasses represented at G, are intended for that purpose ; 

 the joint upon which they act allowing them to be turned 

 down over the sloping side of the prism box. 



At e is shown a spring, which, being pressed by the 

 finger at the time of observation, and then released, checks 

 the vibrations of the card, and brings it more speedily to 

 rest. A stop is likewise fixed at the other side of the box, 

 by which the needle may be thrown off its centre ; which 

 should always be done when the instrument is not in use, 

 as the constant playing of the needle would wear the point 

 upon which it is balanced, and upon the fineness of the 

 point much of the accuracy of the instrument depends. A 

 cover is adapted to the box, and the whole is packed in a 

 case, which may be carried in the pocket without incon- 

 venience. 



The method of using this instrument is very simple. First 



