Trigonometrical Survey. 42 1 
the line of the hooks is made parallel to the line of collima- 
tion in this direction, with the greatest precision. The level 
may be suspended on these pins in the same manner as on the 
horizontal axis. 
The cross wires at N, in the common focus of the object and 
eye-glasses, are fixed at right angles to each other ; but in - 
stead of being placed horizontally and vertically, as in the 
common way, they make each an angle of 45 0 with the plane 
of the horizon. This mode of fixing wires is of the greatest 
advantage in making nice observations, as it remedies the in- 
convenience and error arising from their thickness. To bring 
the line of collimation in the telescope at right angles to the 
horizontal or transverse axis, there are two nuts for the pur- 
pose, one on each side of the box at N, which serve to move 
the intersection of these wires towards the right or left. 
In the eye end of the telescope is a micrometer, which 
serves to measure small angles of elevation or depression. It 
consists of a moveable horizontal wire, placed as close as pos- 
sible to the cross wires already mentioned. By turning the 
micrometer-screw O, this wire is moved across the field of 
the telescope, and the space which it moves through is shown 
in revolutions of the micrometer-screw, by means of an index, 
moveable in a slit, and the divisions on the stem Q. The parts 
of a revolution are shown in tooths by an index P, on the mi- 
crometer head. 
In tracing out a base by intermediate stations, the instru- 
ment must be frequently shifted to the right or left, till the 
telescope shows that the middle of its axis and the extremities 
of the base are in the same vertical plane. To expedite this 
mdccxcv. 3 1 
