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compute the errors. Then if the cross hairs are in the axis of the telescope, the error at c will be double the error at B; and if not, the diaphragm carrying the cross hairs must be moved by means of the screws at c, c, Fig. 104, until the error at c is double the

error at B.

Direction of
Error at C.

A

Upward
Downward

a

B

b

Fig. 163.-Adjustment of Level.

The following table shows the direction in which the cross hairs are to be moved :

Fig. 164.-Adjustment of Level.

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2. To make the Spirit Level parallel to the Collimation Line, or in other words, to ensure that the collimation line is horizontal when the bubble of the level is at the centre of its run.

This is the most important adjustment of the level, as nearly the whole accuracy of the instrument in practice depends on it. The first adjustment need seldom be made, and when making it

care should be taken that the diaphragm screws are screwed up perfectly tight, and it will then seldom require to be interfered with again.

Drive in two pegs at A and B, Fig. 165, or select two good steady marks at these points, and set up the level at c exactly half way between A and B. Then whether the level be in adjustment or not, the staff readings at A and B will be equally and similarly affected, and the difference of these readings will be the correct difference of level of A and B. Now set up the level at d, Fig. 166, as close to a as it is possible to read the staff, and read the staff held on a. Now knowing the correct difference of level between A and B, compute from the last staff reading at a what

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the staff reading at в should be. If now the actual staff reading at B does not agree with this, raise or depress the whole instrument by means of the levelling screws s, s, Fig. 104, until the reading at B is correct. Now bring the bubble to the centre of its run by means of the adjusting capstan screws at e connecting the level with the telescope. See now if the reading of the staff at a is altered, and if not, the adjustment is correct. If the reading at a is altered, the correct reading at в must be again computed from it, and the instrument made to read this by again depressing or raising it by the levelling screws s, s; the bubble being then again brought to the centre of its run by the screws at e, Fig. 104. Unless the level is very much out of adjustment, the alteration of

at A.

the level to the correct staff reading at B will not appreciably affect the staff reading at A, as the distance da is very much less than the distance dB. In any case the second adjustment of the level to the proper staff reading at B will probably not affect the reading The adjustment will therefore, as a rule, be effected at the second trial. When finally adjusted see that both staff readings at A and B are correct when the bubble is at the centre of its run. The adjusting of the bubble by the screws at e, Fig. 104, sometimes moves the level and alters the staff readings, and it is advisable therefore to try both staff readings after moving the screws at e.

3. To make the Telescope and Spirit Level together Perpendicular to the Vertical Axis, or to make the Instrument "traverse,” i.e., so that the bubble remains in the centre of its run in every position of the telescope.

Bring the telescope over one pair of screws and level it up. Now turn it through 180° so as to reverse the ends of the telescope. If the bubble remains in the centre of its run the adjustment is correct; if not, correct half the deviation by means of the screws at t, t, Fig. 104, and half by means of the levelling screws s, s. Repeat the operation until the bubble remains in the centre of its run, when the telescope is reversed end for end. The instrument when levelled over both pairs of screws, or in the three screw instrument over one pair and then over the third screw, will then remain in the centre of its run in every position of the telescope. It is most generally useful to have this adjustment correct when a great many intermediate sights have to be taken, as in cross sectioning; but it is very liable to get out, and seldom remains for long correct. It does not, however, appreciably affect the accuracy of the work; it merely saves time in levelling up the instrument for each sight taken.

As to centring of object glass and eyepiece, horizontality of cross hairs, these adjustments are to be made as described for the theodolite, pages 209, 210.

Focussing the object glass and eyepiece, or adjustment for parallax, comes under the head of temporary adjustments, and are to be made as described for the theodolite, page 203.

Adjustment of the Y Level.-The Y level may be adjusted by twisting the telescope half round on its axis in the Y's, and noting whether the intersection of the cross hairs still continues

to coincide with the same object. If not, half the deviation is corrected by the diaphragm screws. This makes the collimation line coincide with the axis of the Y's, with which the axis of the telescope is supposed to coincide.

To adjust the bubble, level up the instrument and turn it end for end by lifting out the telescope and replacing it. If the bubble deviates from the centre of its run, half the deviation is corrected by the screws attaching the level to the telescope and half the deviation by the levelling screws. This makes the bubble parallel to the Y's, and therefore parallel to the collimation line if the collimation line has been previously adjusted to be parallel to the Y's. Any inequality in the size of the rings on the telescope on which it rests in the Y's will, however, affect the adjustment. See Precise Levelling "pivot correction," pages 192, 193, 195.

It is, however, much more accurate to adjust the Y level by the "peg methods" already described. The Y level is made to "traverse" as already described.

See also instructions for precise spirit levelling, pages 194 to 201, for adjustments of Y level.

CHAPTER V.

RAILWAY SURVEYS AND SETTING OUT.*

Home and Foreign Work. As railway surveys at home are conducted under very different conditions from similar surveys abroad, this chapter is confined to home surveys for railways and to the setting out and incidental work prior to and during construction, reserving for Chapter XI. railway surveys abroad, in jungle, dense forest, and unmapped country.

Parliamentary Surveys.—The preliminary or first survey for a railway is called the Parliamentary survey, and is so called because its object is the preparation of the plans, sections, and estimates for submission to Parliament in order to get its sanction to the proposed railway.

Ordnance Survey Maps. On the Ordnance Survey maps of the United Kingdom it is possible to lay down a line between any two points with a very fair approximation to the best location. The 6 in. Ordnance maps are usually used for this purpose, as contours are given on them, while the 25 in. maps have no contours; the 6 in. scale also conforms to the Parliamentary Standing Orders as to scale of deposited plans.

Special Local Considerations. In home work special local considerations to a very great extent determine the route of the line, which has often to be laid out through expensive cutting to avoid some local landowner's property, &c. &c., while detours have to be made to bring the line within easy reach of towns or villages on the route, or to avoid these when opposition is to be feared, and so on. These matters are usually to be considered in consultation with the promoters of the scheme and the local agent.

Ruling Gradient and Minimum Radius of Curve.The next point is to fix upon the ruling gradient and minimum radius of curve, For these, see pages 230, 231, 232.

*

Practically all this chapter also applies to Road Surveys and Setting Out. For surveys abroad see Chapter XI.

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