SURVEYING INSTRUMENTS. 349 



by dust, or other exposures, that other forms of construc- 

 tion are liable to in this arrangement the lower plate 

 moving within the upper one, and only seen through the 

 openings and glasses; they are completely protected. 

 The glass covering the verniers is the finest plate-glass, 

 and is no obstruction in accurately reading the divisions. 

 The graduated circle and verniers are silvered. The 

 compass-box is 5J inches in diameter. Within the com- 

 pass-box, a graduated circle, divided into 360 degrees, 

 and subdivided to half degrees, and numbered from north 

 and south points each way from to 90. On the face of 

 the compass are engraved the principal points of the com- 

 pass ; in the centre of the box is screwed a steel pin, on 

 Avhich is poised a delicate magnetic needle having a 

 jewelled centre ; at the side of the compass U, there is a 

 small milled-head screw, by turning which, a lever is 

 raised and the needle lifted from its support ; by this the 

 fineness of the point is preserved when not required for 

 use. The parallel plates, Y and G, are held together by 

 a ball and socket, at D, and are set firm and parallel by four 

 milled-head screws. These turn in sockets fixed to the 

 upper plate, while their bases press against the upper 

 side of the lower plate, and being set in pairs opposite to 

 each other, they act in contrary directions. The instru- 

 ment by this means is set up level for observation. Be- 

 neath the parallel plates is a screw adapted to the staff- 

 head, H, which is connected with brass-work, securely 

 jointed to three mahogany legs, about four and a half 

 feet long, square at the joint but round below ; these when 

 spread support the instrument at a proper height for use. 

 In the cut only the upper portion of these legs are repre- 

 sented. The lower plate or limb can be fixed in any 

 position by tightening a clamp-screw, which causes a col- 

 lar to embrace the socket and prevent its moving ; but it 

 may be moved in any small quantity by means of a slow 

 motion or tangent-screw, S, when required to be set in 

 any more exact position than the hand alone could set 

 them. In the same manner the upper or vernier plate 

 may be clamped to the lower, and a slow mytion given 

 by the tangent-screw, R. On one side of the vernier 

 plate is a spirit-level, and at right angles on the supports 

 of the telescope is another ; these levels have at their 

 ends adjusting screws by which they may be accurately 



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