103 



The above reasoning assumes that the deviation of the plane of motion'from the 

 vertical plane is constant or can be maintained constant during a set of obser- 

 vations. This, however, is far from being assured in ordinary instruments. The 

 position of the plane of motion depends upon the accuracy of the adjustment and 

 upon the degree of sensitiveness of the front plate-level, or of the striding- 

 level. Their sensitiveness should therefore be such that the plane of motion shall 

 not deviate from the vertical plane by an amount inconsistent with the degree of 

 precision expected of the instrument. That is to say, levels whose sensitiveness is 

 suitable for an instrument reading to minutes would be insufficient in sensitiveness 

 for an instrument reading to ten seconds. We have seen cases where the more 

 ordinary Engineers' Transits were provided with front plate-levels which would hardly 

 indicate five minutes of arc in an instrument reading to thirty seconds. With such 

 an instrument the plane of motion of the line of collimation cannot therefore be con- 

 trolled within five minutes of arc, from a true vertical plane and all horizontal angles 

 measured by it are influenced by the uncertainty arising from this defect. 



The importance of knowing the degree of sensitiveness of the levels of the finer 

 instruments is very great and on this account, it is usual for us to send with such in- 

 struments a statement giving the value of the divisions of the levels in seconds of arc. 

 The sensitiveness of the transverse striding-level is made much greater than that 

 of the plate-levels, and in such instruments the latter serve principally to level up 

 approximately. Its greater sensitiveness is of especial importance in Mining transits, 

 as steep sighting is the rule in mining surveying. 



The Adjustments of the Arc, or of the Full Vertical Circle with 

 Double Opposite Verniers. 



In the Engineer's Transits provided with an arc or full vertical circle, the ad- 

 justment of its verniers to read zero when the telescope and plates are level should 

 be made as described on page 50, in order to make the instruments serviceable for 

 reading vertical angles. 



When the vertical circle is, however, provided with double opposite verniers, as 

 shown in No. Ic, Style 0, page 147, or as in Tachymeter No. Ig, the adjustment 

 of the vernier zeros for a normal position should be made by the two opposing 

 capstan-headed screws attached to the vernier frame ; but if the telescope is revers- 

 ible over the bearings also, requiring a separate tangent screw, as shown in our 

 Universal Mining Transit No. 8, and in Transit Theodolites No. lla and No. lib, 

 then the adjustment of the vernier zeros for position must be made by the vernier 

 frame's tangent screw. See " Instructions for Using our Universal Mining Transit," 

 paje 177. 



If the vertical circle has a separate level attached to the frame carrying the 

 double opposite verniers, as shown in Mining Transit No. 8, and in Transit-Theod- 

 olites Nos. lla and lib, etc., then the adjustment of this level and verniers for 

 position must be made as described on page 177, so of which we will repeat here 

 much as properly belongs to this subject. 



Place the telescope in the horizontal plane by means of its tangent screw, then 

 move the vernier frames' tangent screw until the zero line of the double verniers, 

 marked A, is in coincidence with the zero line of the vertical circle, and now raise 

 or lower the adjusting screw of this level, as the case may be, until the bubble is 

 in the center of its tube. 



It. is now supposed that the zero line of the double opposite verniers, marked 

 B, is also in coincidence with .that of the vertical circle. If not, the verniers 

 marked B can be moved after releasing the capstan-headed screws, until both zero 

 lines on that side of the vertical circle are also in cbincidence. However, this is a 

 very laborious proceeding for those uninitiated in this work, and as it cannot 

 always be made quite exact, owing to the mode of mounting the vertical circle on 

 the telescope's axis, it will be found easiest to eliminate errors of excentricity 

 in the graduation of the vertical circle and verniers by reversing the telescope and 

 taking the mean of the readings. The vertical circle is graduated from to 90 

 and back, and the verniers are double, so that angles of elevation and depression 

 can be read with ease and dispatch. 



