equipment in the Naval Construction Force Tableof Allowance (TOA). These precision instrumentsare essentially like the conventional levels exceptfor added features.A precision level is one that is equipped withan extra-sensitive level vial. The sensitivity of alevel vial is usually expressed in terms of the sizeof the vertical angle the telescope must be movedto cause the bubble in the level vial to move fromone graduation to the next.The sensitivity of the level vial on an ordinarylevel is about 20 sec. On a precise level, it is about2 sec. The telescope level vial on an ordinarytransit has a sensitivity of about 30 sec.The more sensitive the level vial is, the moredifficult it is to center the bubble. If the level vialon an ordinary level had a sensitivity as high as2 sec, the smallest possible movement of the levelscrew would cause a large motion of the bubble.For this reason, a precise level is usually alsoa tilting level. On a tilting level, the telescope ishinged at the objective end so the eyepiece endcan be raised or lowered. The eyepiece end restson a finely threaded micrometer screw that canbe turned to raise or lower the eyepiece end insmall increments. The instrument is first leveled,as nearly as possible, in the usual manner. Thebubble is then brought to exact center by the useof the micrometer screw.Military LevelThe military level (fig. 11-26) is a semi-preciselevel designed for a more precise work than theengineer’s level. The telescope is a 30-power,10-in.-long, interior-focusing type with aninverting eyepiece and an enclosed fixed reticle.The reticle is mounted internally and cannot beadjusted as in other instruments. It contains crosswires and a set of stadia hairs. The objective isfocused by an internal field lens through a rackand pinion, controlled by a knob on the upperright-hand side of the telescope. The telescope andlevel vial can be tilted through a small angle inthe vertical plane to make the line of sight exactlyhorizontal just before the rod reading is made.The tilting is done by a screw with a graduateddrum located below the telescope eyepiece. A camis provided to raise the telescope off of the tiltingdevice and to hold it firmly when the instrumentis being moved and during the preliminary45.749Figure 11-26.-A military level.11-30
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