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of the bridge at A, which could easily be recognized from description at any future time, if ever it should be necessary to refer to this spot again; it therefore answered as a bench mark.) The surveyor must next set up his spirit-level in the most suitable spot which presents itself, from whence he can have an uninterrupted view, not only of the staff at the back station, but also for a considerable distance in the direction he wishes to carry his levels. The station selected should not in any case exceed four or five chains, and if it be only half that quantity, there will be less likelihood of error; for when long sights (as they are usually termed) are taken, unless both the back and forward stations are equally distant from the instrument, errors will gradually creep in upon the results, which, in a long series of levels, are liable, by their accumulation, to become of serious consequence. The proper station being determined upon, and the tripod legs of the instrument spread out and thrust into the ground sufficiently to insure its stability, the observer must adjust his level for observation in the following order :First, he must draw out the eye-piece of the telescope till he sees the cross wires perfectly well defined; then, directing it to the staff, he must turn the milled-headed screw, on the side of the telescope, till he can likewise distinguish with the utmost possible clearness the

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* It must be borne in mind, when we thus minutely detail what may appear to the practical man as naturally obvious, that we are writing for the information of those who have never had any practice whatever.

smallest graduations on the staff ; that these two adjustments be very carefully and completely performed, is of more consequence than is generally supposed, for upon them depends the existence or nonexistence of parallax. If any parallax is detected, it must be removed, or the observations will be incorrect; its existence may be detected by the observer moving his eye about at the same time that he is looking through the telescope at the staff; and if he sees that the cross wires do not appear to have the least motion with regard to the divisions with which they are coincident, then no parallax will exist; but if any motion appears to take place between the wires and the staff, it is a proof that one or both of the foregoing adjustments have been imperfectly made.

To remedy this inconvenience the eye-piece should first be moved to try and improve the distinct appearance of the cross wires. The observer will be greatly assisted in this operation if he holds a sheet of white paper before the object glass, which, at the same time that it prevents other objects from attracting his attention, presents a clean white disk, or ground, for the wires to be seen upon; and when he is satisfied that they are as sharp and well-defined as possible, he must repeat the movement of the milled head by the side of the telescope till he is equally satisfied of the distinct appearance of the graduations on the staff; then let him again move his eye about before the eye-glass to see if any parallax still exists, and if so, he ought to

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repeat the above simple operation until it is removed. We have known the parallax of a telescope to be a source of great annoyance to persons in the profession, which has led us to be thus minute upon what to some would appear very simple. We have for the like reason given an explanation of its nature, &c., at page 23.

The turning the mill head to obtain distinct vision of the staff, in the old construction of instruments, communicated motion to the object glass; but in those of recent contrivance, it moves the whole of the eye end of the telescope, and with it the cross wires. In either case, the distance between the object glass and the wires is increased to a proper extent; the modern contrivance appears to be the most approved. The adjustment of the eye-piece for distinct vision when once made, is not likely to require alteration the whole day, unless it be accidentally deranged; but that of obtaining distinct vision of the distant staff (together with the one we shall next describe) must be performed at every station, as it varies with the distance of the staff, as explained at page 27.

Having made the above adjustments perfect, bring the spirit-bubble into the centre of its glass tube, which position it must retain unmoved in every direction of the instrument; or, in other words, the bubble must indicate a true level during the time the telescope is turned completely round horizontally on its staff head; this is accomplished by bringing the telescope successively over each pair of the parallel plate screws, and

giving them motion, screwing up one while unscrewing the other to a corresponding extent; but if the telescope is supplied with a cross level, as in that contrived by Mr. Gravatt, the two bubbles, being at right angles to each other, will at once show which pair of screws require turning, in order to produce an indication of level in both bubbles. In the Treatise on Mathematical Instruments there is given an ample explanation of the adjustment of levels in all their details; upon such subjects we shall once for all refer to that work.

Having adjusted the level for observation, it must be directed to the back staff, of which a clear view must be had; then note with all possible exactness the foot, and decimal fraction of a foot, with which the central part of the horizontal wire appears to be coincident, which enter in the proper column of the field or observation book. This column should be headed "Back Sight," or "Back Station," as in the example given at page 45. As soon as it is registered, look to see that the spirit-bubble has not removed from its central position, and then repeat the observation, to insure that no mistake had been made in noting it; this should be invariably done, to guard against errors.

The back observation being made, turn the telescope round in the forward direction, and obtain a distinct view of the staff, by turning the milled head at the side of the telescope; then look at the spirit-bubble, and if it has at all changed its position, by receding towards

either end of its tube, bring it back to the centre by the parallel plate screws, as before described (this can be done so readily, and without moving the telescope, when a cross level is attached, and having likewise other advantages, that we recommend its universal application to spirit-levels); then, by looking through the telescope, observe what division on the staff is intersected by the cross wire, and enter the reading in the proper column of the field-book, which should be headed "Fore Sight," or "Fore Station." Having entered it, look to see that the bubble is still correct, and then verify the observation by noting it again, which will complete the first levels.**

It may be worth remarking that, in setting the level up, the pointed legs should be pushed into the ground sufficiently to insure the stability of the instrument, and likewise that the observer should move himself about the instrument, whilst taking the levels, as little as possible, taking care not to strike the legs with his feet. Caution in these matters is required, for sometimes the least movement of the person will derange the levels of the instrument, particularly on loose or elastic ground; to do away the inconvenience arising from this source, a reflector has been contrived to fix on the top of the telescope tube, by which the observer can see both the staff and the reflected image of the

* When taking levels for the formation of a section, it is sometimes necessary to note the bearing of the compass needle, and to measure distances, as will be explained hereafter.

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