The self-leveling level (also called automatic level) shown in figure 11-27 is a precise, time-saving development in leveling instruments. It did away with the tubular spirit level, whose bubble takes time in centering as well as in resetting its correct position from time to time during operation.

The self-leveling level is equipped with a small bull s-eye level and three leveling screws. The leveling screws, which are on a triangular foot plate, are used to center the bubble of the bulls-eye level approximately. The line of sight automatically becomes horizontal and remains horizontal as long as the bubble remains approximately centered. A prismatic device called a compensator makes this possible. The compensator is suspended on fine, nonmagnetic wires. The action of gravity on the compensator causes the optical system to swing into the position that

Figure 11-27.-An automatic level.

defines a horizontal sight. This horizontal line of sight is maintained despite a  slight out of level of the telescope or even when a slight disturbance occurs on the instrument.