Surveying - Carpenter's and Builder's Math, Plans, and Specifications - page 213

Figure 6-7 Typical transit.

Parallel Plates
There are two plates , one upper and one lower. The lower plate
A is generally formed with two parts. The outside part is a flat
ring and is screwed to the tripod head. The inside part is another
flat ring of a diameter larger than the opening in the outside part
and has a central dome, C , that is perforated on the top. The in-
side part is movable and rests on the under side of the outside
part. The upper plate B is generally made in the form of a cen-
tral nut, with four arms at right angles (or three at 120 ◦ ). The
upper plate carries an inverted conical shell, the lower portion of
which passes through the perforation in the dome of the inside part
of the lower plate, where it expands into a spherical shape and
thus forms a ball joint with the lower plate. This spherical mem-
ber is perforated in the center to allow the passage of a plumb-bob
string.

Foot Screws
The two plates are connected by four-foot (sometimes only three)
screws F in order to clamp the lower and upper plates (making
them fast with each other and with the inverted shell) and to serve