Other types of leveling instruments have been incorporated into the SEABEE units. In fact, the self-leveling level has now become standard

Figure 11-24.-A wye level.

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.