SURVEYING INSTRUMENTS. 345 



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

 motion or tangent-screw, 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 motion given by the 

 tangent-screw. On the vernier plate are two spirit- 

 levels, about three inches long, placed at right angles to 

 each other, and having adjusting screws. A frame rises 

 on each side of the compass-box, firmly fixed on the ver- 

 nier plate, which supports the pivots of the axle of the 

 vertical arc, or semicircle, on which the telescope is 

 placed ; one end of the axis has a clamp and a tangent- 

 screw, by which a slow motion is obtained. The divis- 

 ions on the arc are into thirty minutes, and reading by 

 the vernier plate to single minutes. The level is sup- 

 ported under and parallel to the telescope attached to it, 

 by screws having at one end lateral motion, and at the 

 other parallel motions, by which the level may be prop- 

 erly adjusted to the optical axis of the telescope. The 

 telescope has two collars of bell-metal, truly cylindrical, 

 on which it rests in its supports, and is confined in its 

 place by the clips, which may be opened by removing the 

 pins, for the purpose of reversing the telescope, or giving 

 it a motion on its axis for adjustment. The eye-piece is 

 of the best quality, having four lenses. The object is 

 seen upright, and not reversed as in many instruments, 

 and has a slide by which it can be drawn out for adjust- 

 ment to the eye of the observer ; also four screws for ad- 

 justing the axis of the eye-piece, to coincide with the 

 axis of the object-glass. In the focus of the eye-piece is 

 placed two spider lines, the one horizontal, and the other 

 vertical, fixed in a frame adjustible by four screws. The 

 object-glass is of the best achromatic kind, attached to a 

 moveable tube that may be thrust out, by turning a milled 

 head, to which a tooth and pinion movement is connected ; 

 by this the object-glass may be adjusted to its focus, and 

 show the objects in view distinctly. A brass plummet 

 and line are usually had with the instrument, which is 

 suspended from a hook under its centre, by which it can 

 be placed exactly over the station whence the observa- 

 tions are to be taken. The instrument is securely packed 

 hi a mahogany case. Price $130.00. 



