40 



ANGULAR SURVEYING 



The transit generally has a magnetic needle and a graduated 

 needle circle C, and can therefore be used as an ordinary com- 

 pass. The line of sight, however, instead of being given by a 

 pair of sights is defined by the axis of the telescope. The 

 telescope revolves in a vertical plane on the transverse axis a, 

 and is supported by the standards D. These are attached to 

 the upper, or vernier, plate U. The lower plate carries a 

 graduated circle called the horizontal limb. These plates 

 rotate independently around the vertical axis of the instru- 



FIG. 2 



ment. The vernier plate rotates within and above the other, 

 anil to the former are attached two verniers v that travel along 

 the graduated circle of the lower plate. The vernier plate can 

 be clamped firmly to the lower plate by means of the clamp 

 screw K, called the upper, or vernier, clamp; and by means 

 of the upper tangent screw t it can be revolved slowly on the 

 lower plate, moving the vernier along the divided circle, so 

 that the instrument can easily be set at any given angle. The 

 upper plate is attached to an accurately turned and slightly 

 conical axis or spindle Q, Fig. 2, that extends down nearly to 



