ADJUSTMENTS OF THE THEODOLITE. 
33 
the object with the intersection of the telescope hairs, and take the 
reading of the vertical circle. The mean of these two readings (face 
right and face left) will be the trne reading to which the vernier of the 
vertical arc must be set, by the tangent-screw W. Then by means of 
the clip screws ZZ again cover the object with the intersection of the 
telescope hairs. This operation should be repeated until the reading of 
the vertical circle is the same with the telescope in both positions. When 
this has been accomplished, the bubble of the level on the vernier arms 
must be brought to the middle of its run by the capstan-headed 
screws Y Y at the end of the level-tube. 
The method of ascertaining the value of the divisions of the level scale, 
and of applying the correction for dislevelment to the vernier angles, is 
as follows *— 
By means of the clip screws move the bubble up to one end of its run, 
say towards the object end, so that the object end of the bubble corre¬ 
sponds approximately with the extreme reading of the scale. Intersect 
with the horizontal wire some convenient object for observing. Read 
and record one end of the bubble, say the object end, and the vertical 
angle. Now, by means of the clip screws, bring the bubble back towards 
the eye as far as you can, taking care that it is really floating, and within 
the graduations of the scale. Reintersect the same object as before, and 
record the vertical angle and the reading of the object end of the bubble 
in its new position. The difference between the two readings of the 
object end of the bubble gives the dislevelment in terms of divisions of 
the scale, and the difference between the two vertical angles gives the 
same dislevelment in minutes and seconds of arc. Dividing this angular 
measurement by the number of divisions of dislevelment, you obtain the 
value of one division of the scale in arc. 
Thus :— 
Elevation. Object end of bubble. 
1st observation . . . 7 0 3 ' 28" 18 divisions 
2nd ,,...700 5 
Difference . 
o 3 ‘ 28 
13 
Value of one division = 
13 
= 16" 
* This method is taken from ‘ Text-Book of Military Topography,’ Fart II., 1828. 
VOL. I. D 
