leveling. An eyepiece, located to the left of thetelescope, is used for viewing the bubble throughthe prism system that brings both ends of thebubble (fig. 11-13) into coincidence.The level vial is located directly under thetelescope, but to the left and below, directly inline with the capstan screws under the bubble-viewing eyepiece. The level vial’s sensitivity isgiven as 30 sec per 2-mm spacing. A circularbubble that is viewed through a 450 mirror isprovided for the first approximate leveling beforethe long level vial is used. For night work,battery-powered electric illumination lights thelong bubble, the reticle, and the circular level. Theclamping screw and the horizontal motion tangentscrew are located on the right-hand side; theformer near the spindle and the latter below theobjective lens. The instrument has a three-screwleveling head. The tripod for this level has anon-extension leg to add rigidity and stability tothe setup.Self-Leveling LevelThe self-leveling level (also called auto-matic level) shown in figure 11-27 is a precise,time-saving development in leveling instruments.It did away with the tubular spirit level, whosebubble takes time in centering as well as inresetting its correct position from time to timeduring operation.The self-leveling level is equipped with a smallbull’ s-eye level and three leveling screws. Theleveling screws, which are on a triangular footplate, are used to center the bubble of thebull’s-eye level approximately. The line of sightautomatically becomes horizontal and remainshorizontal as long as the bubble remains approxi-mately centered. A prismatic device called acompensator makes this possible. The com-pensator is suspended on fine, nonmagnetic wires.The action of gravity on the compensator causesthe optical system to swing into the position that45.749AFigure 11-27.-An automatic level.11-31
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