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



the edge of the upper plate, or 180 apart, a short space, a, 

 is also chamfered, forming with the edge of the lower plate 

 a continued inclined plane ; these spaces are likewise covered 

 with silver, and form the verniers. The lower limb is usu- 

 ally graduated to thirty minutes of a degree, and it is sub- 

 divided by the vernier to single minutes, which being read 

 off by the microscope, E, half or even quarter minutes can 

 easily be estimated. 



The parallel plates, F and G, are held together by a ball 

 and socket at D, and are set firm and parallel to each other, 

 by four milled head-screws, three of which, b b b, are shown 

 in the figure : these turn in sockets fixed to the lower plate, 

 while their heads press against the under side of the upper 

 plate, and being set in pairs, opposite each other, they act 

 in contrary directions ; the instrument by this means is set 

 up level for observation. 



Beneath the parallel plates is a female screw adapted to 

 the staff head, which is connected by brass joints to three 

 mahogany legs, so constructed that when shut up they form 

 one round staff, secured in that form for carriage by rings 

 put on them ; and when opened out they make a very firm 

 stand, be the ground ever so uneven. 



The lower horizontal limb can be fixed in any position, by 

 tightening the clamping screw, H, which causes the collar c 

 to embrace the axis, C, and prevents its moving ; but it be- 

 ing requisite that it should be fixed in some precise position 

 more exactly than can be done by the hand alone, the whole 

 instrument, when thus clamped, can be moved any small 

 quantity by means of the slow-motion screw, I, which is 

 attached to the upper parallel plate. In like manner the 

 upper or vernier plate can be fixed to the lower, in any posi- 

 tion, by a clamp (in the plate this clamp is concealed from 

 view), which is also furnished with a slow motion, the screw 

 of which is generally called the tangent-screw. The motion 

 of this limb and of the vertical arc, hereafter to be described, 

 is sometimes effected by a rack and pinion ; but this is 

 greatly inferior, where delicacy is required, to the slow 

 motion produced by the clamp and 'tangent-screw. 



Upon the plane of the vernier plate, two spirit-levels, d, 

 d, are placed at right angles to each other, with their proper 

 adjusting screws ; their use is to determine when the hori- 

 zontal limb is set level ; a compass also is placed at J. 



The frames K and L support the pivots of the horizontal 



