ANGULAR SURVEYING 41 



the tripod head. In transits of the most modern construction, 

 this axis revolves within a socket that is controlled by the 

 leveling screws 5 and about the upper portion of which revolves 

 a socket that extends down from the lower plate, forming what 

 is called a compound center. The centers, which control the 

 entire instrument above the leveling screws, can be clamped 

 against rotation by means of the clamp screw K', and the 

 instrument can then be revolved slowly by means of the tangent 

 screw t'. This clamp screw is called variously the lower clamp, 

 clamp to the centers, or clamp to the lower plate, and the 

 tangent screw is designated by corresponding terms. The cen- 

 ters are connected with plate O, sometimes called the lower 

 leveling plate, by means of a hemispherical or ball-and-socket 

 joint, shown at M. The centers and the entire instrument 

 above them are supported in position by the four leveling 

 screws, which serve also to level the instrument. The plate O 

 screws on the tripod head. This plate and the leveling screws, 

 considered together, are sometimes spoken of as the leveling 

 head. 



The graduated circle is numbered in various ways, three 

 systems of numbering being employed. These may be de- 

 scribed as the azimuth system, in which the figures extend 

 from continuously around the entire circle to 360; the transit 

 system, in which the figures extend from in opposite direc- 

 tions through the adjacent semicircles to 180 at the point 

 diametrically opposite the zero point; and the compass system, 

 in which the figures extend each way from two points dia- 

 metrically opposite each other through the adjacent quadrants 

 to the 90 points. 



There are usually two rows of figures extending around the 

 graduated circle of a transit, each row being numbered accord- 

 ing to one of the preceding systems. In some transits, both 

 rows are of the azimuth system, extending in opposite direc- 

 tions around the circle. The different systems are also com- 

 bined in various ways. 



THE VERNIER 



A vernier is an auxiliary scale used for measuring fractional 

 parts of the smallest subdivisions of the min scale. The 



