104 Mr T. Stevenson 071 Levelling Instruments, 



provements in the arrangement of the various parts of the 

 instrument ; and Mr Gravatt has of late years added a cross- 

 bubble for facilitating the rough-setting of the instrument — 

 or that adjustment which is made with the legs of the tripod ; 

 and an enlargement of the diameter of the object-glass, so as, 

 by the admission of a greater number of rays of light, to allow 

 of the telescope being shortened, without impairing its optical 

 powers. 



A B is the telescope. 



C C the compass-box. 



M the screw for adjusting the focus. 



H H the tubular spirit-level. 



G the spherical or circular level. 



D D D the parallel plate-screws of Sisson. 



K the old ball and socket motion. 



F the new ball and socket motion. 



N clamping-screw for ditto. 



