420 The Account of a 
makes it serve two purposes ; one for determining points in 
the same vertical plane ; the other to show how much a mea- 
sured line deviates from the level. It consists of a telescope 
about eighteen inches long, with an achromatic object-glass 
of about i t 6 q inches diameter. The telescope passes through 
an axis in the manner of a transit, and as it must be used for 
viewing objects at very different distances, the images from 
the object-glass will vary in the same proportion ; it therefore 
becomes necessary to vary the distance of the wires, so that 
they may be exactly in the same place with the image. For 
this purpose there is a pinion, moveable by turning a milled 
head at -A, whereby the small tube, with the wires which are 
contained in the box B, are made to approach, or recede from 
the object-glass. 
The two pivots, or extremities of the axis, are made with 
great accuracy to the same diameter ; and they turn in angles 
in the uprights C and D. Each of the angles is fixed in a 
slider ; one at D, to move horizontally, by turning a finger- 
screw E ; the other vertically, by turning the finger-screw F. 
The level G is here represented as suspended by its hooks on 
the transverse axis. Its use is to shew when that axis is hori- 
zontal ; and it is furnished with an adjusting-screw H, by which 
the two hooks may be made exactly of the same length, so that 
the axis on which it is suspended may become parallel to a 
tangent to the middle of the glass tube. This level also serves 
to set the fine of collimation in the telescope horizontal ; for 
which purpose there are tvyo pins, K and L, attached to the 
side of the telescope, parallel to the axis thereof : one of these 
pins is furnished with an adjusting-screw M, by which the 
