101 



rection in the most precise work. Such deviations, while never existing in our new 



-imients, are, as a rule, caused by unequal wear of the collars, or by an unequal 

 distribution of weight of parts attached to the horizontal axis of revolution, or by 

 au imperfect adjustment of the vertical wire for collimation or by all three com- 

 bined. Errors from this source can also be eliminated by the usual method of 

 reversing the instrument anu telescope and taking the mean. 



The above test can also best be performed by the use of collimators arranged 

 in the vertical plane, see " The Collimator Apparatus " when of course the modus 

 operandi is the same as just described. 



Instead of being obliged to find or make a mark at tne base of a building in 

 order to test the correctness of the vertical adjustment of the wyes a true vertical 

 plane may be had by the use of an artificial horizon. 



Artificial Horizon. This may be made of mercury placed in a shallow bowl of 

 copper or iron (see page 194) or it may be made of molasses, or of oil mixed with 

 finely powdered lamp black ; or it may be made of plane parallel glass suitably 

 mounted on three leveling screws, so that it can be placed in a truly horizontal posi- 

 tion by means of a sensitive spirit-level. In the absence of any of these a bucket 

 full of water may serve the purpose, although the slightest air currents will cause 

 a ripple in the water and make observations more difficult. 



To test the vertical adjustment of the wyes with the aid of an artificial horizon 

 proceed thus: 



Having previously verified the adjustments of the striding-level and instrument 

 set it as near to an elevated object as can be conveniently observed, place the arti- 

 ficial horizon within 8 or 10 feet in front of the instrument ; level up carefully and 

 bring the bubble of the striding-level to the center of its graduation. Now, if with 

 plates clamped, the vertical wire bisects both the object and its reflection in the 

 artificial horizon, it is proof that the line of collimation lies in a truly vertical plane ; 

 but if not, remove half the difference by the wye adjustment and repeat until cor- 

 rect. Now make a memorandum of the deviation of the bubble of the striding-level 

 as explained above in order to apply it as a correction in precise work. 



While, with the use of an artificial horizon, it is not actually necessary, it will 

 be well to repeat this test with the telescope in reversed position to see if the object 

 and its reflected image remain bisected. If not, it will indicate that the instrument 

 has not been properly adjusted before the test was made, 



Test of the Equality of the Cylindrical Pivots of horizontal axis of revolution by means 

 of the transverse striding-level resting at circle of contact in wyes. Carefully level up the 

 instrument by the plate level. Next verify the adjustments of the transverse strid- 

 ing-ievel as explained on page 96 and then make the wye adjustment of the standards 

 by reversing the instrument on its vertical- center and, removing half the difference if 

 any and repeating this operation until the bubble of the striding level remains hi the cen- 

 ter of its graduation. Now remove the striding-level, and upon reversing the telescope's 

 axis of revolution end for end in the bearings and placing the striding-level again on the 

 pivots, so that each end will be on the same side of the standard previously occupied be- 

 fore the telescope was reversed in the bearings, see if the bubble comes back to its 

 center. If so, the pivots are of equal diameter, but if not, remove half the difference by 

 the wye adjustment and note to what extent and towards which pivot the bubble of the 

 striding-level moves and make a memorandum of it so that it may be applied as a cor- 

 rection in precise work. 



The test of the equality of the pivots at the circle of contact in the wyes when 

 the telescope is reversible over the bearings may also be made by any of the methods 

 explained in the preceding paragraph for verifying the adjustment of the line of col- 

 lima"lou for exact motion in the vertical plane. In this case, after the horizontal 

 axis has been adjusted for the vertical plane with the telescope in the direct posi- 

 tion, it is only necessary to reverse the telescope over its bearings, without unclamp- 

 ing the plates, and to repeat the test for motion in the vertical plane. If correct 

 the pivots are of equal diameters ; but, if not, the inequality amounts to one-half 

 of the deviation of the line of collimation from the vertical plane, and for best 

 results in very steep sighting, it will be well to reverse the telescope for this pur- 

 pose and to accept the mean of the two readings as the true result. 



On the other hand, however, if the telescope is intended only for reversal 

 through the standards and turning the instrument on its vertical center, as all our 

 Transit-Theodolites do, then for good results after the cylindrical pivots are worn, 



