SURVEYING INSTRUMENTS. 

 Fig 99. 



81 



the vernier plate, A. Their form is conical, nicely fitted 

 and ground into each other, having an easy and a very steady 

 motion ; the external centre also fits into a ball at D, and the 

 parts are held together by a screw at the lower end of the 

 internal axis. 



The diameter of the lower plate is greater than that of 

 the upper one, and its edge is chamfered off and covered 

 W'th silver, to receive the graduations : on opposite parts of 



