Seven-feet Transit Instrument. 429 
and when observations by direct vision, are to be compared 
with those obtained by reflection, the index of the one, will 
point the telescope to the direct place of the star, whilst that of 
the other, will present the instrument to its reflected image. 
Figures 3 and 4, of Plate XVI. exhibit the side pieces and Y* 
in which the pivots of the axis rest ; the plates which are semi- 
circular, are imbedded in the stone piers, and are firmly 
screwed into them. Figure 3, represents the eastern plate, in 
which the adjustment for the level of the axis is made : a 
piece, of which the upper end is formed into a Y, is moveable 
perpendicularly, but well secured from motion in every other 
direction ; the means of gradual adjustment are brought about, 
by a piece having a short cylindrical part in the middle, at the 
upper end a fine screw, and at the lower end a coarse one ; 
the fine screw works in the moveable piece, and the coarse 
one in the fixed plate ; the cylindrical part being perforated 
in many places, enables it to be acted upon by a capstan pin, 
and thus an effect equal to the difference of the two screws, 
is produced. This last part, because easy of description, was 
not brought under the view of the draftsman, by removing 
the covering plate ; a slit in it however exposes two or three 
of the capstan holes of the differential screw. 
Fig. 4, Plate XVI. shows the western plate, the general out- 
line of which corresponds with that just described; the motion 
of the Y piece is here only horizontal, for the purpose of plac- 
ing the instrument in the meridian. The adjustment is effected 
by means of two screws, which work in the opposite sides 
of the moveable Y piece, and whose heads abut against the 
fixed plate To produce motion in the Y piece, one of them 
must be screwed, and the other unscrewed ; but in order 
mdcccxxvi. 3 K 
