THE PLANE-TABLE. 



81 



the inner surface of one and the outer surface of the other being ground so 

 as to fit accurately to one another. The inner cup is in two parts, or rather 

 consists of two rings one outside the other, the one controlling the move- 

 ment in level and the other that in azimuth. From each of these rings there 

 projects beneath the movement a screw, and upon each of these screws is a 

 nut by which it is clamped. There is no tangent screw for either the leveling 



a. Plane Table board f. UpperLevel Cup 



b Bearing Plate g. Longer " 



c. Tripod Seaa h . Level Clamp 



et. • • Legs i ^zimutfi Clamp 

 e.JjzimuOiCup 



Fig. 8.— Johnson plane-table tripod head. Section. 



or the azimuth motion, as none is required. The movement is sustained 

 by a light hard-wood tripod with split legs. The board used generally 

 accommodates a full atlas sheet, but necessarily differs in size, owing to 

 the different scales of Held work adopted. The largest board used for 

 this movement holds an atlas sheet upon a scale of 1:45000, and is 24 by 36 

 inches in size. 



MON XXII (J 



