18 



HYDKOGRAPHICAL SURVEYING 



[chap. I. 



Adjust- 

 ment for 

 Vertical 

 Limb. 



Adjust- 

 ment for 

 Vertical 

 1 Icne. 



Points 

 liable to 

 Derange- 

 ment. 



" that the two parts of the axis have been inaccurately ground, 

 " and the fault can only be remedied by the instrument maker."* 



To adjust for the vertical limb, the bubble of the level being 

 in the centre of its run, reverse the telescope, end for end, in 

 the Y's, and if the bubble does not remain in the same position, 

 correct for one half the error by the capstan-headed adjusting 

 screw at one end of the level, and for the other half, by the 

 vertical tangent screw. Repeat the operation till the result is 

 perfectly satisfactory. Next turn the telescope round a little, 

 both to the right and to the left, and if the bubble does not still 

 remain in the centre of its run, the level must be adjusted 

 laterally by means of the screw at its other end. This adjust- 

 ment will probably disturb the first, and the whole operation 

 must then be carefully repeated. By means of a small screw, 

 fastening the vernier of the vertical limb to the vernier plate 

 over the compass box, the zero of this vernier may now be set 

 to the zero of the hmb, and the vertical limb mil be adjusted 

 for it horizontally. 



The vertical limb should move in a truly vertical plane. 



Any error can only be adjusted in the larger instruments, 

 but every theodolite must be tested for it, as, if much error 

 exists, the instrument requires alteration by the maker. 

 It will introduce error into all angles to objects much 

 elevated or depressed, and it is especially important for 

 observations for true bearing to know that this adjustment 

 is perfect. 



To test it, direct the theodolite when horizontal to either 

 the edge of a well-built wall, or still better, a steady plumb- 

 line. The cross-wires, when the instrument is elevated and 

 depressed, should still intersect the line. 



If they do not, in 6-inch theodolites, the adjustment can 

 generally be made by means of screws on one of the Y frames. 

 In smaller theodolites we must accept the error, and take care 

 not to use them for true bearings. 



These adjustments completed, the instrument will be ready 

 for work. 



There are, however, a variety of small points on which a 

 theodolite may go wrong while away in the field, and a know- 

 ledge of the general causes of these temporary derangements 



* From Heather. 



