38 



ingenious application of friction wheels and the lever. There 

 is also an ingenious contrivance for adjusting the axis hori- 

 zontal. 



The circle of brass is divided into intervals of 5 minutes, 

 which intervals are subdivided by micrometer microscopes 

 into seconds and parts of a second as usual. 



There are three microscopes. One called the bottom mi- 

 croscope, opposite the lowest part of the circle : a second 

 opposite the left extremity of the horizontal diameter, and 

 a third opposite the right extremity of the horizontal dia- 

 meter. 



The frame carrying the circle turns on the vertical axis 

 with the greatest steadiness. The circle also turns on the 

 horizontal axis with equal steadiness. 



The vertical axis of the instrument is adjusted by a, 

 plumb line. The plumb line which performs this adjust- 

 ment is about 10 feet long, and is suspended from a point 

 about 8 inclies from the centre of the top of the frame, and 

 passes over a point below, 8 feet from the point of sus- 

 pension. By help of this point which is moveable by a 

 screw, and by the moveable socket below, the axis of the 

 instrument is made vertical. The adjustment of this axis, as to 

 the north and south positions is, it is evident, of the most es- 

 sential consequence to the exactness of the zenith distance 

 of the object observed. It is likewise evident that, from the 

 great interval between the upper and lower parts of the in- 

 strument, the temperatures above and below must occasion- 



