MATHEMATICAL AND SURVEYING INSTRUMENTS. 71 
point in the scale is then to be read off. This upward and downward motion 
takes place, as shown in fig. 52, by means of ropes and pulleys. Pulleys, a 
and b, are attached at the top and bottom of the beam AB, over which the 
endless string cd passes. E, the carrier of the vane or target, is fastened to 
the string at F, with which it must move. The sight vane may have very 
different constructions; the great object of all, however, is to show the 
horizontal line with the greatest possible distinctness at considerable dis- 
tances. Thus in fig. 51 there is a broad black stripe at AB, through whose 
centre runs a white horizontal line. The vane has an opening through 
which the point of elevation of the central horizontal line can be ascertained. 
Other vanes are four cornered or circular, and divided into quadrants which 
are alternately red and white, or black and white; others are arranged as 
in fig. 53, where AB, the central line, is drawn black on a white ground. 
Here abcd is the arrangement for fastening the target to the staff. The 
same is seen at NOPQ (jig. 54), on the hind side of the target. The target 
is screwed to the sliding piece. In levelling with targets of the construc- 
tion just described, the central line is marked on the hind side, and the 
graduation of the staff is also posterior. The perforated vanes (fig. 51) are 
better, however, as the leveller can frequently read off through his telescope, 
and thereby control the operations of the vanesman. 
