ADJUSTMENT, USE, AND CADE OF INSTRUMENTS. 83
;en looking
S *isfc
e<i oncefcral
r indirect, method
;cond method does
considerable angle
iis particular), a«]
t, being careful to
)f course the toil
il elevation, He
level polygon, li
ero, notwithstand-
rel over the sane
qual, and note the
nt and there «re
)
in length, andset
e profile indepen-
on the same piece
il scale should he
he tracing. Each
in length. Estab-
:nt determine the
■d and back, See
run.
nee would be ch
in levelling; that
too low. Why is
elling ?
CHAPTER IV.
INSTRUMENTS FOR MEASURING ANGLES.
THE TRANSIT.
86. The Engineer’s Transit is the most useful and
universal of all surveying-instruments. Besides measuring
horizontal and vertical angles it will read distances by means
of stadia wires, determine bearings by means of the magnetic
needle, do the work of a solar compass by means of a special
attachment, and do levelling by means of a bubble attached
to the telescope. It is therefore competent to perform all the
kinds of service rendered by any of the instruments heretofore
described, and is sometimes called the “universal instrument.”
A cut of this instrument is shown in Fig. 17. Fig. 18 is a
sectional view through the axis of a transit of different
manufacture.
The telescope, needle-circle, and vernier plates are rigidly
attached to the inner spindle which turns in the socket C>
Fig. 18. This portion of the instrument is called the alidade,
as it is the part to which the line of sight is attached. The
socket C carries the horizontal limb, shown at B, and may
itself revolve in the outer socket attached to the levelling-head.
Either or both of these connections may be made rigid by
means of proper clamping devices. If the horizontal limb B
be clamped rigidly to the levelling-head and the alidade spindle
be allowed to revolve, then horizontal angles may be read by
noting the vernier-readings on the fixed horizontal limb for
the different pointings of telescope. If the horizontal limb
itself be set and clamped so that one of the verniers reads zero
