CHAPTER IV. ADJUSTMENT OF INSTRUMENTS. Chain. From pulling through fences or other causes the chain is liable to become lengthened or shortened, and must be tested and adjusted from time to time. The best way to test the chain is to compare it with a steel tape, noting also whether the half-chain or 50 links mark is exactly 33 ft. If the chain is too short, it must be lengthened by straightening out any links which may have become bent, or by inserting one or two of the spare rings connecting the links, some of these being always provided on the chain for this purpose. If the chain is too long, some of these rings must be taken out. A Government standard of all kinds of English measures has been established in Trafalgar Square, London, by means of permanent bronze marks let into the granite plinth of the terrace wall in front of the National Gallery. There is also a standard in the Guildhall belonging to the Corporation of London, and similar marks are established in nearly every city or town by the burgh surveyor, in some central position. Theodolite. The adjustments of the theodolite may be divided into the Temporary Adjustments and the Permanent Adjustments. TEMPORARY ADJUSTMENTS.—These are the adjustments which must be attended to each time the instrument is set up. 1. To Set up the Instrument over a Station. This is effected by means of the plumb bob and plumb line attached to the vertical axis of the instrument, which is brought over the point first roughly by means of the legs and then exactly by the adjusting screws or sliding plate. When there are no adjusting screws or sliding plate the plumb bob is brought over the point by means of the legs only, pressing into the ground one leg or another until the plumb bob is exactly over the point. In accurate work see that the levelling up of the instrument does not disturb the position of the plumb bob, and if so, readjust and level up again. 2. To Level up the Instrument. By this is meant making the horizontal plates 7 and y, Fig. 52, truly horizontal, and the vertical axis truly vertical. It is effected by means of the two spirit levels v attached to the upper or vernier plate . Rotate the instrument horizontally until one of the levels v is parallel to one pair of the levelling screws g'. By means of these screws bring the bubble of this level to the centre of its run, and by the third screw, or if there are four levelling screws, by means of the other pair of screws, bring the bubble of the other level to the centre of its run. Now rotate the instrument horizontally through 180°, and see if the bubbles of the two spirit levels still remain in the centre of their runs. If they do, the adjustment is effected; but if not, then the spirit levels are not parallel to the upper plate t. In this case correct half the deviation by means of the capstan screws attaching the levels to the upper plate, and half the deviation by the levelling screws g', and repeat the operation until the bubbles remain in the centre of their runs while the instrument is rotated into any position. The plates and y should be clamped together by the screw a' during this adjustment. As this adjustment is so liable to get out, it is here described under the head of temporary adjustments, although the adjustment of the levels themselves parallel to the plate t might be called one of the permanent adjustments; the levelling up by means of the screws g' being strictly speaking the temporary adjustment. A more exact adjustment may be made by means of the large level attached to the telescope. Set the vernier of the vertical circle to zero and bring the telescope over one pair of screws. Now level it up by the levelling screws g'. Turn the instrument horizontally through 180°, and if the bubble of the telescope level deviates from the centre of its run, correct half the deviation by the tangent screw of the vertical circle and half by the levelling screws g. Repeat the operation until the bubble remains in the centre of its run, when the instrument is turned through 180°. Now turn the horizontal plates and y (clamped) through 90° so that the telescope is over the third screw, or when there are four levelling screws, over the other pair of screws, and again level up the telescope level by the levelling screws g'. The horizontal plates and y will then be horizontal and the vertical axis vertical. 3. Focussing the Object Glass and Eyepiece, or as it is called, adjustment for parallax. The foci of the object glass and eyepiece glasses must be made to coincide with the cross hairs of the diaphragm. To adjust the eyepiece b, look through the telescope, and push the eyepiece out or in until the cross hairs are seen clearly and distinctly. Then direct the telescope to some well-defined object, and by means of the milled-headed screw at the side of the telescope move the object glass a out or in until the image of the object is seen clearly and apparently coinciding with the cross hairs of the diaphragm. The adjustment of the eyepiece need only be made once, but the object glass must be focussed for each new object sighted on at different distances. A good test for parallax is to move the head slightly from side to side. If the adjustments are correctly made, the image will appear to coincide steadily with the cross hairs; if not, it will appear to move from side to side, as the head is moved. It is to be corrected by pulling the eyepiece out or in and focussing the object glass as already described. PERMANENT ADJUSTMENTS. 1. Adjustment of the Supports of the Horizontal Axis of the Telescope.--The object of this adjustment is to make the horizontal axis of the telescope truly level when the instrument is levelled up, so that the telescope may revolve in a truly vertical plane. Direct the telescope to a well-defined object of considerable altitude, such as the finial on the top of a church spire, the theodolite being placed close to the bottom of the church. Direct the cross hairs on to the top of the spire, clamping the horizontal plates and y, Fig. 52. Now depress the telescope to the bottom of the church, and there mark a point bisected by the cross hairs. Unclamp and reverse the position of the supports by turning the instrument through 180° horizontally or in azimuth, and again direct the cross hairs to the top of the spire. Clamp again, adjust exactly on to the top of the spire, and see whether the cross hairs still cut the same point previously marked on the bottom of the church when the telescope is depressed. If not, mark where they cut, and also mark the point midway between the two marks. This middle point is the correct point in the same vertical plane with the top of the spire. Correct quarter of the total deviation between the first two marks by the screws ss at the top of the support and quarter by the tangent screw d' of the horizontal circle, thus bringing the cross hairs on to the middle point. Repeat the operation until the middle point and the top of the spire are both bisected in both positions of the supports, when the adjustment will be correct. Here we observe that the collimation line is again directed exactly on to the top of the spire before being directed to the bottom of the church for the second time. No error other than that due to the supports is therefore introduced, the telescope being merely depressed. This adjustment may also be tried by seeing if the cross hairs bisect an object of considerable altitude and its image as seen reflected in the horizontal surface of a fluid. 2. Adjustment of the Collimation Line.-The adjustment of the line of collimation consists in making it exactly perpendicular to the horizontal axis about which the telescope rotates. Let the cross hairs be directed on to some well-defined distant object P, and bring them to coincide exactly with it. In Fig. 158, let aob represent the horizontal axis of the telescope, and cod the collimation line directed as described on to the point P. If now the collimation line be perpendicular to the horizontal axis, when the telescope is lifted out of its bearings and replaced with the ends of the horizontal axis ab reversed into the position ba, Fig. 159, the collimation line cd will still be directed on to P. If, however, the collimation line cd be not perpendicular to the horizontal axis ab, as shown in Fig. 160, when the ends of the horizontal axis are reversed into the position ba, Fig. 161, the collimation line will no longer be directed on to the point P, but on some other point as P'. In this case the point " midway between P and P' is the correct position of the collimation line perpendicular to the horizontal axis ab. The method of adjustment is therefore as follows:Direct the cross hairs on to some distant point P, clamping the upper and lower plates by the screws a' and ', Fig. 52, and leaving the vertical arc unclamped, so that the telescope may be easily lifted out of its bearings. Now carefully lift the telescope out of its bearings and replace it with the ends of the horizontal axis |