CHAP. I.] THEODOLITE 17 



azimuth error. When this is done, the cross-Avires, while the 

 telescope is slowly revolved, should remain over the object. 



It should be noticed that in loosening one coUimating screw Action of 

 and tightening the other, the arm of the " tommy " moves in making 

 the same direction for both screws ; one screw should be loos- Screws, 

 ened before the other is tightened. 



Using the inverting tube, if it is required to move the cross 

 of the wires upwards, the upper screw must be loosened and the 

 lower one tightened. Attention to these rules will save an in- 

 experienced hand much trouble, and wear and tear on the 

 threads of the screws. 



The collar being tightened by its clamping screw, unclamp Adjust- 

 the vernier plate, and turn it round till the telescope is over ™®"^.*^°^ 

 two of the parallel plate screws. Bring the bubble of the level tai Limb 

 beneath the telescope to the centre of its run by turning the 

 tangent screw of the vertical arc. " Turn the vernier plate half 

 " round, bringing the telescope again over the same pair of the 

 " parallel plate screws ; and, if the bubble of the level be not still 

 " in the centre of its run, bring it back to the centre, half-way, 

 " by turning the parallel plate screws over which it is placed, and 

 " half-way by turning the tangent screw of the vertical arc. 

 " Repeat this operation till the bubble remains accurately in the 

 " centre of its run in both positions of the telescope ; and then, 

 " turning the vernier plate round till the telescope is over the 

 " other pair of parallel plate screws, bring the bubble again to 

 " the centre of its run by turning these screws. The bubble will 

 " now retain its position, while the vernier plate is turned com- 

 " pletely round, showing that the internal azimuthal axis, about 

 " which it turns, is truly vertical. 



" If the bubbles of the levels on the vernier plate are now 

 " brought to the centre of their tubes, by means of the screws 

 " fitted for the purpose, they will be adjusted to show the verti- 

 " cality of the internal azimuthal axis. 



" Now, having clamped the vernier plate, loosen the collar, by 

 " turning back the screw, and move the whole instrument slowly 

 " round upon the external azimuthal axis, and, if the bubble of 

 " the level beneath the telescope maintains its position during 

 " a complete revolution, the external azimuthal axis is truly 

 "parallel with the internal, and both are vertical at the same 

 " time ; but if the bubble does not maintain its position, it shows 



2 



