Surveying - Carpenter's and Builder's Math, Plans, and Specifications - page 225

Do not grease or oil the slide too freely; only a thin lubricant
film is necessary. Any surplus of oil should be removed with a
clean wiper.
The centers, subject to considerable wear, require frequent
lubrication. After a thorough cleaning, they should be care-
fully oiled with fine watch oil. All of the adjusting screws
should be brought to a fine bearing, but they should never
be tightened to such a degree that a strain is applied to the
different parts. If this is done, the adjustment will be unreli-
able.
When the instrument is carried on the tripod, all clamps should
be tightened to prevent unnecessary wear on the centers.

The Stadia
This is a device that is used for measuring distances, and it consists
essentially of two extra parallel hairs in addition to the ordinary
crosshairs of the transit or a level telescope (see Figure 6-12). The
stadia hairs may be adjustable, or they may be fixed permanently
on the diaphragm.
When using the stadia, distances are measured by observing
through the telescope of a transit the space (on a graduated rod)
included between two horizontal hairs (called stadia hairs ). If the
rod is held at different distances from the instrument, different in-
tervals on the rod are included between the stadia hairs. The spaces

F

C F

Figure 6-12 The principle of stadia operation. The fixed sta-
dia hairs are set so that they will intercept 1 foot on a rod
at a distance of 100 feet. Since the image of the crosshairs is
projected to a point beyond the telescope objective equal to
its focal length, the rays of light converge at that point, and
measurements must begin from there. Therefore, a constant
must be added to all stadia readings equal to the focal length
of the object lens, plus the distance from the face of the objec-
tive to the center of the instrument. This constant is the fac-
tor F + C. For transit telescopes, it is equal to approximately
1foot.