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 +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 



