90 A MANUAL FOR NORTHERN WOODSMEN 
adjustable end of the level tube is raised or lowered by 
means of the adjusting screws until the bubble is in the 
middle. The adjustment should then be correct, but it 
will be well to test it. 
EXAMPLE 
Instrument at A 
Rod reading on A = 4.062 
Rod reading on B =5.129 
Diff. elev. of A and B = 1.067 
Instrument at B 
Rod reading on B = 5.076 
Rod reading on A = 4.127 
Diff. elev. of B and A = 0.949 
Mean of the two results = 1.067 -f- 0.949 = 1.008, true diff. in elev. 
2 
Instrument is now 5.076 above B. 
Rod reading at A should be 5.076 — 1.008 = 4.068 to give a level 
sight. 
This method of adjustment may be used for the transit 
with this difference — that instead of adjusting the level 
tube to the line of sight, the level tube is first made hori¬ 
zontal and then the line of sight is made parallel with it 
by adjusting the cross-hair. The same is true of a dumpy- 
level. 
(d.) Make the axis of the level tube perpendicular to 
the vertical axis of the instrument. 
Bring the two clips down over the telescope and fasten 
them. Level the instrument, bring the bubble precisely to 
the middle of the tube over one set of levelling screws, and 
then turn the telescope 180° about the vertical axis. If 
the bubble moves from the center, bring it half-way back 
by means of the adjusting screws at the foot of one of the 
wye supports. 
Since the bubble is brought to the center of the tube each 
time a rod reading is taken, this last adjustment in no way 
affects the accuracy of levelling work, but it is a con¬ 
venience and a saving of time. 
2. Use of the Level 
Levelling is employed to get the difference in elevation 
between points. With the level set up and the rod held on 
