SURVEYING INSTRUMENTS. 83 



axis of the vertical arc, or semicircle, M, on which the tele- 

 scope is placed. The arm which bears the microscope, N, 

 for reading the altitudes or depressions, measured by the 

 semicircle, and denoted by the vernier, e, has a motion of 

 several degrees between the bars of the frame, K, and can 

 be moved before the face of the vernier for reading it off. 

 Another arm clamps the opposite end of the horizontal axis 

 by turning the screw, 0, and has a tangent-screw of slow 

 motion at P, by which the vertical arc and telescope are 

 moved very small quantities up or down, to perfect the con- 

 tact when an observation is made. 



One side of the vertical arc is inlaid with silver, and 

 divided to single minutes by the help of its vernier ; and 

 the other side shows the difference between the hypothenuse 

 and base of a right-angled triangle, or the number of links 

 to be deducted from each chain's length, in measuring up 

 or down an inclined plane, to reduce it to the horizontal 

 measure. The level, which is shown under and parallel to 

 the telescope, is attached to it at one end by a joint, and at 

 the other by a capstan-headed screw, /, which, being raised 

 or lowered, will set the level parallel to the optical axis of 

 the telescope, or line of collimation ; the screw, g, at the 

 opposite end, is to adjust it laterally, for true parallelism in 

 this respect. The telescope has two collars, or rings, of 

 bell metal, ground truly cylindrical, on which it rests in its 

 supports, h h, called Y's, from their resemblance to that 

 letter; and it is confined in its place by the clips, i i, which 

 may be opened by removing the pins, j j, for the purpose 

 of reversing the telescope, or allowing it a circular motion 

 round its axis, during the adjustment. 



In the focus of the eye-glass are placed three lines, 

 formed of spider's web, one horizontal, and two crossing it, 

 so as to include a small angle between them ; a method of 

 fixing the wires which is better than having one perpendi- 

 cular wire, because an object at a distance can be made to 

 bisect the said small angle with more certainty than it can 

 be bisected by a vertical wire. The screws adjusting the 

 cross wires are shown at m : there are four of these screws, 

 two of which are placed opposite each other, and at right 

 angles to the other two, so that by easing one and tighten- 

 ing the opposite one of each pair, the intersection of the 

 cross wires may be placed in adjustment. 



The object-glass is thrust outwards by turning the milled 



