TOL. IXI.] PHILOSOPHICAL TRANSACTIONS. 105 



the middle of the upper surface of the horizontal plate, is placed a ground glass 

 level, by which the plate is set parallel, and the pillar perpendicular to the hori- 

 zon; from this plate rise perpendicularly two quadrants, one of which is divided 

 for the latitude into half degrees, and has a vernier index to 3 minutes; the equa- 

 torial plate, with its hour circle, is supported by the two quadrants; its axis of 

 motion, which is placed near the hours xii, xii, passes through the centres of 

 the quadrants, and carries the index i, pointing to the divided quadrant ; the 

 equatorial plate is divided into half degrees, and has a vernier index showing 

 every 3 minutes of right ascension, or 12 seconds of time ; it is figured to show 

 both degrees and time, to prevent misapprehension, it may be right to remark, 

 that the hours xii, xii, ought properly to have been placed according to the 

 meridian line ; they arc here placed otherwise, for the convenience of better 

 seeing the meridian distance shown by the vernier; on the upper part of the equa- 

 torial plate is a plate, on which are fixed two supporters, which support the axis, 

 under which is fastened the semicircle of declination, divided into half degrees, 

 and has a vernier index subdividing it to 3 minutes; on the upper part of this 

 axis, is fixed an achromatic telescope, which magnifies about 50 times; to the 

 eye end of this telescope, is applied a small reflecting speculum, making an angle 

 of 45° with the axis of the telescope, by which objects that are in the zenith, or 

 any other altitude, may be observed, without putting the body in any inconveni- 

 ent position ; to the under part of the axis n, is fastened a brass arm carrying 

 the weight a, which counterbalances the telescope, and the brass work, annexed 

 to it; while 2 weights counterbalance in like manner the whole of the instru- 

 ment that is moveable on the equatorial axis; so that whatever position the instru- 

 ment is put in, it will there remain, being perfectly balanced; the 4 motions of 

 this instrument may, when required, be moved extremely slow, by means of the 

 indented edges of the circle and semicircles, and the screws or worms to which 

 the handles are fixed, viz, that for the horizontal motion, called the horizontal 

 handle, the handle of latitude, the equatorial handle, and the declination 

 handle. 



To adjust the instrument for observation, the first thing to be done is to make 

 the horizontal plate level, by means of the spirit level, and the 3 adjusting 

 screws at the bottom of the stand ; this being done, move the equatorial plate 

 either with or without the latitude handle, until the index on the quadrant points 

 to the latitude of the place ; and then the equatorial plate will be raised to the 

 elevation of the equator of the place; which is equal to the complement of the 

 latitude ; and thus the instrument is ready for observation. 



VOL. XIII. 



