the wye adjustment should be made only with the telescope in its direct and proper 

 position in the instrument and by the striding-level alone, that is, if the latter rests 

 directly on the pivots as shown in No. 116. The telescope must then not be reversed 

 over the bearings. 



In Transit-Theodolites having no Btridlng-level th<Mvye adjustment of the stan- 

 dards, after the pivots are worn, must be made only with the telescope in dhvri 

 position as explained aboveand as explained on page 51 for the Engineers' transit. 



If the Transit-Theodolite has a striding-level resting on special collars (:- 

 of these instruments are made by us for the sake of greater compactness and 

 portability) and the pivots and collars have become unequal in diameter from wear 

 and tear, then the instrument should also be treated as if it was non-reversible over 

 the bearings and the wye adjustment must be made with the telescope in the direct 

 position only by any of the methods given for the vertical adjustment of the wyes, and 

 after completion of the adjustment this difference in the reading of the striding-level 

 should be noted, etc., for correction as already explained. 



Of course in making any of these tests for equality of collars, pivots and vertical 

 adjustment of the wyes, it is of the utmost importance to have the instrument balanced 

 as the maker intended it to be, as without this none of the above tests can be made. 

 It will not do for instance, to detach the vertical circle after the wye adjustment has 

 been made with it on the instrument, nor will it do, if a solar attachment or a side 

 telescope, etc. has been attached to the telescope's axis of revolution, to suppose that 

 the counterpoises made for each can be indiscriminately interchanged or left off at will 

 without disturbing the equipoise of the instrument and its adjustments. All these 

 parts bear a strict relation to each other and cannot therefore be changed at will without 

 also deranging other existing conditions of the greatest importance for a successful 

 working of the instrument. 



Appendix to the Description of the Adjustments of the Transverse Striding-Level. 



The above tests have been given in great detail to create a familiarity with 

 this subject, on the part of the Engineer, that he may know the proper method to 

 attain results consistent with his intentions. It is important to realize how much 

 depends upon the degree of sensitiveness of the spirit-levels used, and upon the 

 power of the telescope to reveal slight differences in its pointing. The utility of 

 the striding-level in the finer instruments will be best understood when we consider 

 that the method of reversion, so useful in detecting and eliminating errors of 

 graduation and of collimation, is useless in detecting or eliminating deviations of 

 the plane of motion of the line of collimation from the vertical plane passing 

 through the center of the instrument. The adjustment of the vertical wire (de- 

 scribed or page 49) causes the line of collimation to move in a plane perpendicular 

 to the horizontal axis of revolution, and the vertical adjustment of the standards, 

 described on page 51, renders it possible to place that plane in a vertical position 

 as nearly as the degree of sensitiveness of the front plate-level or the striding-level 

 will permit. The actual position of the plane of motion of the line of collimation 

 being dependent upon the levels, a want of sufficient sensitiveness in them will 

 cause the plane to deviate from the vertical plane paesing through the center of 

 the instrument. The angle between these planes measures the greatest angular 

 amount of deviation that the line of collimation can make with the vertical plane. 

 This occurs when the telescope is pointing vertically upwards or downwards. 

 The angular deviation of the line of collimation from the vertical plane is mil 

 when thetelecope is horizontal and it increases as the telescope is more and more 

 inclined. 



When a horizontal angle is measured between points so situated that the tele- 

 scope does not need to be changed in altitude during the observations, each point- 

 ing deviates from the vertical plane to the same amount and in the same sen^e and 

 the error i8 eliminated from the observation. This, however, is rarely the case, 

 and when the telescope is changed in altitude during a set of observations, a de- 

 viation of the line of collimination from the vertical plane introduces an error 

 which cannot be detected and which cannot be removed by repeating the angle, 

 and which is greatest when one pointing is made with the telescope horizontal 

 and the other with the telescope vertical, which can be done with some mining 

 transits. 



