The engineers level is a widely used instrument for leveling operations. Its sighting device is a 30 3 variable power telescope, with a maximum length of 18 in. and with an erecting eyepiece. Some models use internal focusing, while others use external focusing objective

Figure 11-23.-View of a vertical circle for direct and reversal pointings.

assemblies. The reticle has two cross hairs at right angles to each other, and some models have stadia hairs. The telescope and level bar assembly is mounted on a spindle that permits the unit to be revolved only in a horizontal plane. It cannot be elevated or depressed. A clamp and tangent screw acts on this spindle for small motions to permit accurate centering. The spindle mounts in a four-screw leveling head that rests on a foot plate. The foot plate screws onto the threads on the tripod. When the instrument is properly leveled and adjusted, the line of sight, defined by the horizontal cross hair, will describe a horizontal plane.

The two distinct types of engineers levels, classified according to their support, are the wye level and the dumpy level. The WYE LEVEL (fig. 11-24) is so called because its telescope is supported by a pair of wye rings. These rings can be opened for the purpose of turning the telescope or rotating it around its horizontal axis. The bubble tube (vial) can be adjusted, either vertically or laterally, by means of adjusting nuts at the ends of the bubble tube. All these features are provided for the purpose of making fine adjustments. The DUMPY LEVEL (fig. 11-25) has its telescope rigidly attached to the level bar, which supports an adjustable, highly sensitive level vial. During visual leveling operations and observations, both types handle similar basic operations. Their cross hairs are brought into focus by rotation of the eyepiece, and their target, into clear focus by rotation of the focusing knob. Their telescope can be exactly trained on targets by lightly tightening the azimuth clamp and manipulating the azimuth tangent screw.