Surveying and Levelling Instruments, Theoretically and Practically Described. / For construction, qualities, selection, preservation, adjustments, and uses; with other apparatus and appliances used by civil engineers and surveyors in the field.

by William Ford Stanley

195.—Setting-up of the Y or other Level with Parallel Plates.—The tripod stand is opened out so that the legs stand, if on level ground, inclined towards the centre of the instrument at an angle of about 70° to the horizon. The toes of the legs are each separately pressed into the ground sufficiently to make the instrument stand quite firmly. The instrument is then taken from its case and screwed down tightly upon the tripod head.

196.—The Eye-piece is Adjusted, [art. 108], by sliding it gently in and out until the webs can be seen most distinctly. On a bright day a white pocket-handkerchief may with advantage be thrown singly over the object-glass to prevent any confusion from objects in the field of view during the focussing of the eye-piece. For the setting-up adjustment of the telescope, it is brought in position to lie directly over one pair of parallel plate screws, Fig. 55, PS, SP.

Fig. 56.—Diagram plan of parallel plate screw milled heads.

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197.—The milled heads only of these screws are represented in plan in the diagram Fig. 56, aa′ being the opposite pair over which the telescope will be assumed to be at first placed. The level tube is now brought to adjustment by bringing the bubble to the centre of its run by means of the parallel plate screws aa′, by taking the milled heads of these screws, one between the ball of the thumb and forefinger of each hand, and rolling them simultaneously the one in one direction and the other in the reverse. This action tips the axis of the telescope in one direction or the other. Thus by the screws being rolled inwards, as shown by the direction of the arrows in the diagram, the left-hand side of the instrument would be raised. If turned the reverse way, the right hand end would be raised. The opposite end, from that to which the bubble runs, always requires to be raised. Where the ground is rather soft, adjustment when nearly correct may be made partially by pressing down one or other of the legs; in this case the telescope should be placed parallel with the toe of the leg which is pressed down and the axis of the instrument.

198.—When the level tube is adjusted over the screws aa′ it is then placed over bb′ and adjusted in a similar manner, returning again to the position aa′ for final adjustment. When the level is in perfect adjustment the bubble should stand in the centre of its run in making a complete circuit of the horizon by rotation of the instrument upon its vertical axis.

199.—In and during the setting-up adjustment it is most important that the screws should not be made tight enough to cause, by their pressure upon the parallel plates, distortion of the vertical axis. Should this occur, the instrument will not level in all positions by the same setting. The action of the screws also, from the great elasticity of the metal, should distribute the pressure about equally between the opposite pairs aa′ and bb′. The difficulty of accomplishing this with certainty makes another form of adjustment, with three screws only, preferable for setting-up, which will be considered further on. Where the instrument is set up for use, if the adjustment of the bubble be fairly correct to the centre of its run, the reading of the staff may be sighted and the telescope brought to true focus upon it by moving its milled head until the divisions of the staff are as sharp as possible, and then moving the eye upwards and downwards to be sure there is no error of parallax, [art. 109]. After this the final adjusting of the bubble should be made, noting particularly that there are the same number of divisions in its run on each side from the centre if it is a divided bubble.

200.—Adjustment of the Axis of the Telescope in true parallel direction with the periphery of its supporting collars in its Y's. This is performed entirely with the four capstan-headed screws which adjust the diaphragm, one of which is shown, [Fig. 54], C. Having the adjustment of the eye-piece in focus for the webs in the manner described, [arts. 108, 109], the object-glass focussed upon a distant distinct small object or mark, and without parallax, the instrument which carries the telescope is then exactly adjusted to make the intersection of the webs cut the mark. The telescope is now turned half round on its axis, so that the lower part becomes the upper, and observation is again made of the distant small object or mark. If the same intersection of the webs falls on the same point of the object, the collimation adjustment is perfect. If it does not do so, the upper capstan-headed screw at C, or the under opposite one, is loosened by means of the small pin provided with the instrument, and the opposite screw tightened until the webs are brought over a point situated half-way between the points cut by the first and second observation. The telescope is again directed to the point first observed, and the adjustment checked to see if it has been done correctly, that is, if the level reverses, cutting the same point, or whether it requires further adjustment by the same process as before. The other web of the diaphragm, at right angles to the first, is adjusted in a similar manner, but with the other pair of capstan-headed screws.