the other point and repeat the operation. This will give the difference of elevation 

 between the points minus or plus this error. The mean of these two values will 

 therefore be free from this error. Since we now know the true difference of elevation 

 between the points we may establish a level line. The transit is at a known height 

 above one of the bench marks, so we can calculate the rod reading necessary to put the 

 target on the same level with the instrument. After this is done set the horizontal 

 wire on the target and bring the bubble to the center by means of the adjusting screws. 



If it is desired to leave the bubble undisturbed, the cross-hair may be moved by 

 the top and bottom adjusting screws until in the required position. It is not advisable 

 to do this, however, as it would move the horizontal wire out of the optical axis and 

 would also disturb the adjustment of the vertical wire. 



Instruments of our make which have erecting eye-pieces are so mechanically perfect 

 that the cross-wires may be placed in the optical axis by simply placing them in the 

 center of the field of view. In instruments having adjustable eye-pieces and in all in- 

 struments having inverting eye-pieces the cross-wires can be placed in the optical axis 

 only by rotating the telescope in improvised wyes. 



EXAMPLE. 



Suppose that the transit is set near A and is found to be "4.204 above it. A sight is taken on B, the rod 

 reading being 2.966. Hence B is apparently 1.238 feet above A. Next the transit is set at B. Height above 

 B is 4.018. Reading on A, 5.180. Hence B is apparently 1.162 feet above A. The mean of 1.238 and 1.162, 

 or 1.200 is the true difference. The transit is now 4.018 above B, consequently it is 4.018 + 1.200, or 5.218 

 above A. If the target is set at 5.218 while on A it will be on the same level as the cross wire and the adjust- 

 ment can then be made. 



If desired the adjustment may be made as follows : 



To adjust the telescope level in the field, setup the transit in the middle of a toler- 

 ably level piece of ground, and carefully level it. At equal distances, in opposite 

 directions from the transit, drive two stakes, so that the readings of a level rod held 

 successively on each of them will be the same when the telescope level bubble is 

 brought to the center of its tube by the vertical tangent screw in each case, and the 

 instrument is turned in azimuth. Take up the instrument and reset it over one of the 

 stakes ; measure the vertical distance from the center of the horizontal axis of the 

 telescope to the top of the stake over which the instrument is set. Set the target of the 

 rod to read this distance. Hold the rod on the distant stake, and bisect the target 

 with the horizontal cross-line. With the target thus bisected, turn the cylindrical 

 nuts at the object-glass end of the level, till the bubble plays in the middle of its tube. 

 Test the adjustment by resetting half way between the two stakes, and noting that 

 the bubble remains in the same position, and the rod gives the same reading when the 

 instrument is turned in azimuth alone upon the two stakes. Sometimes it is con- 

 venient to use a sheet of water for the same purpose. Two stakes are driven in the 

 water-bed at different distances from the transit, until their tops are even with the sur- 

 face of the water. The transit is leveled up near one of them, and its telescope altered 

 in altitude until a rod held on each successively gives the same reading. Then with 

 the telescope clamped in this position, the adjusting nuts are altered as before until the 

 bubble plays in the middle of its tube. The methods of this paragraph assume the 

 horizontal wire to be adjusted for collimation. 



It is the vertical wire, however, which in the transit is the most important. When 

 that is to be alone adjusted in the field, it is usually done according to the following 

 simple directions : Select two distant points in opposite directions from the instrument, 

 preferably in the same horizontal plane, such that the vertical cross-line will bisect 

 them both when the telescope is pointed upon one, and then the telescope is reversed 

 around its horizontal axis. After bisecting the second point selected, revolve the in- 

 strument in azimuth and bisect the first point again by means of the tangent screw. 

 Reverse the telescope around its horizontal axis again, and if the second point is now 

 bisected the adjustment for collimation of the vertical wire is correct. If it is not 

 bisected, move* the vertical wire one-fourth of the distance between its present position 



* To adjust the line of collimation in a telescope showing objects erect, the diaphragm bearing the wires 

 must be moved in the direction in which the error is observed (as if to increase the error); in telescopes show- 

 ing objects inverted the wires must be moved in the direction lessening the error observed. To move the 

 vertical wire, slightly loosen the respective capstan-headed screws at the side of the telescope, and draw to a 

 corresponding degree the one on the opposite side. To move the horizontal wire, make use only of the ver- 

 tical capstan-headed screws on the telescope in the manner described. C. L. B. 



