338 



Improvements in the Surveyor''s Compass. 



a graduation upon it of twenty degrees each side of the meridian 

 line ; the inner limb, on the outside of which is a nonius graduation 

 to five minutes, which comes in contact with the graduation on the 

 outer limb, and on the inside, the usual graduation for the needle, 

 the glass covers both limbs, protecting from the atmosphere and 

 giving the nonius graduation the advantage of being silvered and re- 

 taining its whiteness. In this construction the advantage is gained of 

 being assured that the centre of the limbs, centre of motion of the 

 inner limb and needle, and the centre of the graduations are one 

 and the same.* 



A level is attached to the compass, and a graduation for taking 

 altitudes, by which the practitioner, when his compass is level and 

 fitted to his course, is enabled to take the level or angle of ascent 

 or descent of any object in his course, from his station, without any 

 additional machinery or adjustment ; to effect this, tv/o apertures are 

 made for the eye in one of the sights, one at the top, for looking 

 down hill, and one at the bottom, for looking up hill ; these apertures 

 are the centre of arches of which the other sight is a tangent, and 

 there is a graduation on each side next to the eye to correspond with 

 the degrees of the arches ; by looking through the small aperture, 

 the point of the graduated sight, in a line with the object, gives the 

 true angle required. 



The annexed figure, in connexion with the above description, will 

 sufficiently explain the instrument. The artist is now making ar- 

 rangements to be ready to execute orders at very short notice, for 

 compasses containing all or parts of the improvements, as may be want- 

 ed. Also to graduate the sights of old compasses for taking altitudes. 



fl? 



The figure represents the compass as if dissected by a Une through the centre, 

 a outer stationary limb; b inner moveable limb; c needle; tZ sights; e bar of 

 the compass ; _/* glass. 



AVith a very little additional expense and variation, this instrument may be 

 transformed into a Thcodolile having two pairs of sights, one pair attached to the out- 

 er limbs, which is to be moveable and the inner limb stationary. 



