Surveying - Carpenter's and Builder's Math, Plans, and Specifications - page 212

In the most exact work, the angles are measured with a spe-
cial vertical circle instrument. In less precise work, an ordinary
Theodolite, whose vertical arc reads by verniers to 30 seconds or
to 20 seconds, may be used, but with such instruments, only sin-
gle readings can be made. The best results with such an instrument
are obtained by taking the average of several independent readings,
one-half of which are taken with the telescope direct and the other
half with the telescope inverted. In every case, the index correction
(or reading of the vertical arc when the telescope is level) must be
recorded.
The Transit
This instrument is designed and used for measuring both horizontal
and vertical angles. It consists of a telescope mounted in standards
that are attached to a horizontal plate (called the limb ). Inside the
limb, and concentric with it, is another plate (called the vernier
plate ). The lower plate or limb turns on a vertical spindle or axis
that fits into a socket in the tripod head. By means of a clamp and a
tangent screw, it may be fastened in any position and made to move
slowly through a small arc.
The circumference of this plate is usually graduated in divi-
sions of either one-half or one-third of one degree, and in the
common form of transit, these divisions are numbered from one
point on the limb in both directions around to the opposite point,
which is 180 degrees. The graduation is generally concealed be-
neath the plate above it, except at the verniers. This upper plate
is the vernier plate that turns on a spindle fitted into a socket
in the lower plate. It is also provided with a clamp by means of
which it can be held in any position and with a tangent screw by
which it can be turned through a small arc. A vernier is a device
for reading smaller divisions on the scales than could otherwise be
read.
The transit is generally provided with a compass so that the bear-
ing of any given line with the magnetic meridian may be determined
if desired. It also has a spirit level attached to the telescope, so that
it may be brought to a horizontal position and made to serve as a
level. Figure 6-7 shows a typical transit.
Construction of the Transit
Figure 6-8 shows the general features of the transit construction.
The following sections describe these features.