132 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



lowered in their sockets by means of the milled-headed 

 screws, b b, for a purpose hereafter to be explained. The 

 weight of the axis, with the load it carries, is prevented 

 from pressing too heavily on its bearings, by two friction 

 rollers on which it rests, one of which is shown at e. A 

 spiral spring, fixed in the body of each pillar, presses the 

 rollers upwards, with a force nearly a counterpoise to the 

 superincumbent weight ; the rollers on receiving the axis 

 yield to the pressure, and allow the pivots to find their 

 proper bearings in the Y's, relieving them, however, from 

 a great portion of the weight. 



The telescope, K, is connected with the horizontal axis, 

 in a manner similar to that of the portable transit instru- 

 ment. Upon the axis, as a centre, is fixed the double circle, 

 J J, each circle being close against the telescope, and on 

 each side of it. The circles are fastened together by small 

 brass pillars ; by this circle the vertical angles are mea- 

 sured, and the graduations are cut on a narrow ring of 

 silver, inlaid on one of the sides, which is usually termed 

 the face of the instrument ; a distinction essential in making 

 observations. The clamp for fixing, and the tangent-screw 

 for giving a slow motion to the vertical circle, are placed 

 beneath it, between the pillars, H H, and attached to them, 

 as shoAvn at L. A similar contrivance for the azimuth cir- 

 cle is represented at M. The reading microscopes for the 

 vertical circle, are carried by two arms bent upwards near 

 their extremities, and attached towards the top of one of 

 the pillars. The projecting arms are shown at N, and the 

 microscopes above at O. 



A diaphragm, or pierced plate, is" fixed in the principal 

 focus of the telescope, on which are stretched five vertical 

 and five horizontal wires : the intersection of the two centre 

 ones, denoting the optical axis of the telescope, is the. point 

 with which the terrestrial object is bisected, when observing 

 angles for geodesical purposes. The vertical wires are used 

 for the same purpose as those in the transit telescope, and 

 the horizontal ones for taking altitudes of celestial objects. 

 A micrometer having a movable wire is sometimes attached 

 to the eye-end of the telescope, but it is not generally ap- 

 plied to instruments of portable dimensions. The illumina- 

 tion of the wires at night is by a lamp, supported near the 

 top of one of the pillars, as at d, and placed opposite the 

 end of one of the pivots of the axis, which, being perfo- 



