6 
Description of Mr, South’s 
declination axis, and the two principal reading microscopes, 
viz. those which give declinations. There is a third micro- 
scope, which indicates zenith distances. This is seen in Plate I. 
fig. 1, between the eye end of the telescope and the instru- 
ment’s elevated pole. In the latter figure is shown, on the 
extreme border of the drum, a narrow brass ring, whereon 
the graduation is made. This ring is not only narrow, but as 
thin as was judged consistent with sound workmanship ; this 
slightness is necessary, because iron plate and brass expand 
very differently, but the former being much stronger, the 
latter must obey it: the brass is soldered to the iron, and also 
pinned to it at short intervals. 
The hour circle, two feet in diameter, is fastened to the 
lower end of the polar axis, the edge of which is seen in Plate I. 
fig. i, its under side in Plate II., and its face in Plate III. 
fig. 5. One of the reading microscopes is well seen in Plate I. 
fig. 1 , and both of them, less perfectly in the other two figures. 
The whole of this circle was of brass, and the divisions were 
at first made upon that metal ; but twenty-two years exposure 
to the atmosphere in the neighbourhood of London had oblite- 
rated the graduation. This has been restored by Mr. Trough- 
ton, upon an inlaid ring of platina, the divisions ( fine lines) 
corresponding to twenty seconds each, which are subdivided 
by the microscopes to tenths of seconds. The declination circle 
is divided to five minutes, which are subdivided by the micro- 
meter screw of the microscopes to single seconds, with a ca- 
pability of estimating further subdivisions. The instrument is 
furnished with two good ground levels, neither of which are 
seen in any of the figures. The divided side of the declination 
circle has been called a drum ; but the reverse is articulated ; 
