and move with it ; and besides being very compact and readily manipulated, it en- 

 ables us to give the pivots of the horizontal axis a form by which greater lateral 

 stiffness is secured for the standards, and without this the telescope of these instru- 

 ments would not have the required stability. 



The striding-level of the finest class of instruments, such as Transit-Theodo- 

 lites No. lib, 12, etc., used for triangulation wholly, and Mining .Transit No. 8, used 

 mainly for very steep sighting, will rest directly upon the cylindrical pivots of the 

 transverse axis, at the circle of contact in the wyes. This latter method is preferred 

 for the finest class of instruments, on account of greater simplicity and accuracy, 

 although the striding-level is then more liable to injury, as it cannot be attached to 

 the telescope and must be lifted off whenever the telescope is moved greatly in 

 altitude or when it is to be reversed, and is liable to fall off if the instrument is 

 taken up hastily. 



The transverse striding-level is adjusted precisely as described, on page 96, 

 for the adjustment of the longitudinal striding-level shown on page 180. To test 

 the lateral adjustment, the striding-level may be rocked upon the transverse axis, 

 if it rests upon its pivots ; or, if mounted upon collars between the standards and 

 attached to the telescope, the latter may be moved on its horizontal axis a few 

 degrees up and down. The horizontal adjustment of the striding-level is tested by 

 reversing it end for end. 



The transverse striding-level is the most important level of the transit, when 

 the latter is used for measuring horizontal angles and for ranging straight lines 

 where objects differ greatly in height 



The adjustment of the transverse axis of revolution. The striding-level having been 

 carefully adjusted, level up the instrument generally with the plate-levels, put the 

 striding-level in position and bring its bubble to the center of its graduation by means 

 of the leveling screws, then turn the instrument 180 on its vertical axis and note 

 whether the bubble of the striding-level remains in the center of the graduation. If 

 it does, the adjustment is correct. If it does not, correct one-half the error by means 

 of tne leveling screws, and the other half by means of the wye adjustment of 

 the standard. Repeat the process until the adjustment is correct. Observe also, 

 in adjusting the wye adjustment of the standards, that it will be best performed and 

 more lasting when the last turns of the lower capstan-headed screw are always ap- 

 plied in an upward direction. 



The transverse axis of the telescope is now adjusted for movement of the latter 

 in a vertical plane. The striding-level, being very sensitive, is the best-known de- 

 vice for making the transverse axis of the telescope truly horizontal. The method 

 assumes that the special collars, or the cylindrical pivots at the end of the axis, 

 which rest directly on wye bearings in the standards, as the case may be, are of 

 equal diameters, and if collars are used, that they are concentric with the pivots of 

 the axis. These conditions are best fulfilled when the striding-level rests directly 

 on the pivots of the horizontal axis, as in the finest triangulation transits. 



Whenever exact vertical motion is desired, as when the objects sighted at differ 

 materially in altitude, the striding-level in such instruments should be depended 

 upon to the exclusion of the front plate-level. The latter is then entirely subordi- 

 nate to the striding-level and should be depended upon merely for leveling the in- 

 strument approximately. The plate-levels are also useful in indicating quickly any 

 large disturbances of position. When the objects sighted at do not differ much in 

 altitude, the front plate-level is, in these instruments, sufficiently sensitive to give 

 satisfactory results without using the striding-level. Where no striding-level is 

 provided, the plate-levels are, of course, to be depended upon for all work, and as a 

 rule, in our instruments the degree of sensitiveness of the front plate-level is such 

 as to insure that the plane described by the telescope shall not differ from the true 

 vertical plane by an amount larger than the smallest direct reading of the verniers 

 of the horizontal circle. 



The adjustment of the vertical wire for collimation. The object of this adjustment 

 is to place the vertical wire so that the line of sight shall be at right angles to the 

 horizontal axis of revolution. This operation presupposes that the optical axis 

 is coincident with the geometrical axis of the focussing slide and also that this 

 combined axis moves at right angles to the transverse axis, so that the line of colli- 

 mation shall describe a vertical plane at all focussing positions of the object-slide 

 when the transverse axis has been placed in a horizontal position. In which case, 



