equipment in the Naval Construction Force Table
of Allowance (TOA). These precision instruments
are essentially like the conventional levels except
for added features.
A precision level is one that is equipped with
an extra-sensitive level vial. The sensitivity of a
level vial is usually expressed in terms of the size
of the vertical angle the telescope must be moved
to cause the bubble in the level vial to move from
one graduation to the next.
The sensitivity of the level vial on an ordinary
level is about 20 sec. On a precise level, it is about
2 sec. The telescope level vial on an ordinary
transit has a sensitivity of about 30 sec.
The more sensitive the level vial is, the more
difficult it is to center the bubble. If the level vial
on an ordinary level had a sensitivity as high as
2 sec, the smallest possible movement of the level
screw would cause a large motion of the bubble.
For this reason, a precise level is usually also
a tilting level. On a tilting level, the telescope is
hinged at the objective end so the eyepiece end
can be raised or lowered. The eyepiece end rests
on a finely threaded micrometer screw that can
be turned to raise or lower the eyepiece end in
small increments. The instrument is first leveled,
as nearly as possible, in the usual manner. The
bubble is then brought to exact center by the use
of the micrometer screw.
The military level (fig. 11-26) is a semi-precise
level designed for a more precise work than the
engineers level. The telescope is a 30-power,
10-in.-long, interior-focusing type with an
inverting eyepiece and an enclosed fixed reticle.
The reticle is mounted internally and cannot be
adjusted as in other instruments. It contains cross
wires and a set of stadia hairs. The objective is
focused by an internal field lens through a rack
and pinion, controlled by a knob on the upper
right-hand side of the telescope. The telescope and
level vial can be tilted through a small angle in
the vertical plane to make the line of sight exactly
horizontal just before the rod reading is made.
The tilting is done by a screw with a graduated
drum located below the telescope eyepiece. A cam
is provided to raise the telescope off of the tilting
device and to hold it firmly when the instrument
is being moved and during the preliminary
Figure 11-26.-A military level.