Fig. gj Shwinjf jjeneral equipment with Vertical Clamp, Telescope 



Bubble, Vertical Circle with Cover Guard (open or closed, as desired) 



and one Double Vernier at the top, as covered under the Mo. 45 



<H-in., No. 55 s-in. or No. 65 6-in. Tachymeter 



To o\ercome the effects of temporal displacements in the hori- 

 zontal plates, the vernier, or verniers, of the vertical limb are 

 attached to the cover guard and mounted so that they may move 

 in short arcs concentrically with the vertical circle. On the cover 

 guard is mounted a level vial, as shown in Figs. 38 and 71, which is 

 brought to the center of its run when the requirements expressed in 

 the first paragraph above are fulfilled. When the index of the ver- 

 nier is in the horizon, or nadir, as the case may be, the control 

 bubble is centered with its own adjusting screws. Conversely, 

 when the control bubble is centered, the verniers of the vertical 

 circle will be adjusted to a position to indicate correct vertical angles, 

 irrespective of slight displacements in the rest of the instrument. 



