STANDARD - Survey Instrument Antique Center!

52
Plumbing and Centering Arrangements.
It now remains to speak of several conveniences of the instrument under congideration.
By a simple mechanical contrivance the plumb-bob when suspended
from the instrument can be set immediately at any desired height. It is suspended
directly from the center of the instrument, and not from the tripod head. This
precaution shou 1 '1 T r
be taken >th every instrument, since otherwise, when there is
difficulty in setting up an instrument, and the legs are unsymmetrically placed, the
plumb-line will not pass through the center of the instrument.
The instrument is provided with the shifting tripod, better known as the shifting
center, by means of which, when the plumb-bob of the instrument is within a fraction
of an inch over a point on the ground, it may be brought immediately over it, by
moving the body of the instrument on its lower level plate. This is probably the
greatest time-saving arrangement which modern makers have introduced in engineers'
transits.
1
Shifting- Center for a Transit with Three Leveling Screws.
There are several methods of placing a level or transit with three leveling screws
upon the tripod head. One is the tribrach style where the leveling screws rest in radial
grooves in a triangular shaped foot-plate which screws on to the tripod. This
method is used extensively in Europe, but, as the instrument rests only by its
own weight in these grooves it is liable to changes in position during use and this instability
becomes greatly aggravated when the leveling screws become worn.
Another method in transits is to rest the leveling screws, provided with small bearing
cups or washers at their lower ends, directly upon the smooth top surface of the tripod
head to allow centering of the instrument over a given point. The pressure of a spiral
spring forming part of the instrument fastener is then applied to fix the position of
instrument on the tripod head.
This latter method of connecting a transit to a tripod is very insecure, inasmuch as
the slightest change in the position of the instrument, while reading a series of angles,
will greatly affect the results.
Thus it is seen that a transit lacking
the necessary stability on its tripod
often proves almost useless in the
finer field-work.
Messrs C. L. Berger & Sons make
the leveling screws to rest in radial
grooves in a separate piece made to
slide on the tripod head as shown in
cut. A clamp-nut, provided with a
large flange and handles, serves to
secure this sliding piece to the tripod
in any position in the range of its
lateral motion. The instrument fas-
tener, being part of the tripod proper,
has a large cylindrical hole in the
threaded stem to allow the hook and
chain, suspended
transit center, to
directly from the
pass through and
to swing freely in every direction, so
that when the plumb-bob is attached
its point will be truly in the continuation
of the vertical center of the
instrument. The milled head at the
lower end of the fastener serves to
screw the latter to the instrument,
andamilledheaded nut acting against
a spiral spring secures the instrument
to the tripod. In use, the pressure
of this spring must be sufficient to
take up the back lash or any loose-