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STAFF-HOLDER AND LEVELLING-STAFF.

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mark, then we can refer to it at any time; with reference to levelling operations, this would be a " Bench mark.” Similarly, in going over some miles of levels for a section, or any other object, we are certain to require at some future period some permanently fixed points, the levels of which may at any time be referred to without any matter of doubt; they may be on or off the actual line of section, so that they are not too far, and provided they are to be readily found. Without such marks we should have to look for exact spots on the surface of the ground where the staff had been held, and no reliance could be placed on anything of the kind, in such an important matter as that of a level, where a few inches are certain to be more or less of great importance; but if at the same time that the levels have been taken over a line of country, the levels have been taken at the same time to the sill of a particular door or window, the level coping of a wall, the lower or upper* hook of a gate, the top of a milestone, or horsing-block, the root of a tree where a bench is cut for the levelling-staff to be held, the up or down rail under the soffit of a bridge, at the north or south, east or west side, then we have so many defined points over which the levels may be again taken, and by such means the accuracy of former levels may be and are constantly checked. Therefore it should be remembered, that in taking a line of levels, it is of the utmost importance to take levels to some such bench marks, not at greater distances than a mile apart along the line, and nearer if convenient; if not directly on the line, it is of no consequence ; no distance is measured to them, and they are merely noted as B. M. in the column of distances, and described in the page devoted to remarks. This page is always the right-hand page of the level-book, the lefthand one only being reserved for the various columns of entries such as we have already shown. Bench marks are not to be plotted on the section, as they may be at any time referred to in the level-book. It does not follow from what we have said that such marks are always to be found exactly at the end or beginning of a line of levels; they may be found some greater or less distance off, and either at a higher or lower level than the commencement of the line of section ; suffice it to say, that they must be found somewhere, or else such permanent marks must in some manner be firmly constructed or sunk in the ground. For reasons to be explained hereafter, bench marks should be taken at every road crossed by the line of levels. Statf-holder and Levelling-staff.-We have already described the construction of the levelling-staff; we must now make a few observations as to the method of using it, for as much care is required in doing this as the level itself. In setting up the staff for observation, with its face towards the observer, it should be held firmly on the ground, and perfectly upright; the staff-holder should stand facing the observer; he should hold the staff close to him, with the foot near his toes, and his hands on the sides of the staff, and not with his fingers across the face of the graduations. On level ground there is no difficulty in holding the staff perfectly upright, but on sloping ground it is quite different; if the staff leans at all sideways, it will not be parallel with the vertical wires of the diaphragm, and the observer signals right or left to the staff-holder accordingly; but if it leans from or towards the observer, he will not be able to discern this by the appearance of the staff; but the telescope will give the least reading upon it when it is quite vertical, therefore on sloping ground, when the staff is drawn out to two joints or to its full length of the three, the staff-holder should, as soon as he sees that the observation is being made, lean the staff backwards and forwards towards the observer, when the least reading will be the correct one, for the staff will then be perfectly upright. The observation being recorded, the staff-holder, at a signal made, shoulders his staff, and goes forward to the spot where he is next to set up; the same operation is repeated, and, unless in the case of intermediate levels, the observer moves forward next with his level, and passes the staff-holder to get to the next spot where the level is to be planted. The staff-holder will now have to turn the staff round with its face towards the observer, and it is here that considerable care is required, for a careless man, or one who knows nothing about it, will very likely lift the staff with a clod of earth to it, and set it down again two or three inches from the right place; an inch or two difference of level may thus be made; this repeated occasionally throughout a line of levels, will render the whole of the observations worthless. It will therefore be seen that it is of the utmost importance that the staff be replaced exactly at the same spot, and without any additions in the shape of earth attached to its foot. This deserves far greater consideration than may at first sight appear, in going through ploughed ground for instance. We have said at the beginning that the staff should be pressed firmly on the ground, and if in addition to this the staff-holder gives the staff a side or screwy pressure on the ground, it will prevent the earth from adhering to the brass-plate at the foot. It will now be observed that if the observer and his staff-holder are some eight or ten chains apart, a kind of freemasonry must be established between

* The lower hook is generally preferred.

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STAFF-HOLDER AND LEVELLING-STAFF.

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them, for it is only by signals that the observer can make himself understood by the staff-holder, and therefore a set of intelligible signals must be established between them, about the meaning of which there can be no mistake, and which as the work proceeds the observer will have an opportunity of repeatedly explaining as he and the staff-holder pass by each other.

When taking a set of levels to which more than ordinary care is being given, it constantly occurs that the horizontal wire intersects the staff more or less half way between the different small divisions on the staff, when either thousandths must be recorded, or the staff-holder must be instructed to give the staff a smart rap on the ground, which will set this right by bringing the staff a little lower down.

There is one particular precaution which, being carefully enforced, secures the accuracy of position of the staff for fore and backsights; this is a thin substance of some kind placed on the ground for the staff to rest upon, as then when the staff-holder turns round for the next backsight, it indicates where the staff is to be held. A triangular piece of sheet-iron, with the corners turned down and a stud in the middle, with a bit of chain and ring attached to carry the whole, has been used ; but it is rather awkward for the man to carry when dragging through hedge and

litch. We always use a large penny-piece, or a bit of slate, and satisfy ourselves that it is used by occasionally looking under the staff as we pass along; if neglected, a fine generally sets the matter right.

Enough has been said to show the importance of a staffholder's duties; and although men of humble position are generally employed for this work, when they are intelligent, it is surprising what a little trouble taken with them at the onset will do in fitting them for their work. A good staff-holder will not only give but little trouble as to the manner he holds his staff, but he will know exactly where to set it up,—he will understand signals readily, and often be useful in looking out for benchmarks. The amount of time saved by this means is often surprising

When taking levels along a line of country having but little inclination, it is not necessary that all three joints of the staff be kept drawn out, two will be sufficient. This considerably diminishes the staff-holder's labour, which must be fast, by decreasing the leverage, from the lesser length of staff resting on his shoulder.

Along a line of country where long sights may be taken, a very considerable amount of time may be saved by using two levelling staves instead of one ; but care is required in doing this, and it should not be resorted to until the engineer or sur

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and so on;

veyor has got accustomed to his work ; indeed it is only when he has got into the habit of levelling that he can do so with safety. In the first place he must ascertain not only that his staves are of equal length, but also that they are equally divided. In practice it is carried out in the following manner :-At Fig. 132, let one staff-holder set up a staff at A, and let the other staff-holder set up the other at B; as soon as backsight A is taken, a signal is made, and A runs on C, whilst the foresight B is read; the observer moves on to 2, looks to see that C is in the right place, plants the level, and reads off backsight B; he then signals B, who moves on towards D, whilst the foresight C is being read,

but for intermediate levels the two staff-holders would only lead to confusion, therefore as soon as we had read off B, we should send him on towards F, and only make use of A to hold the staff consecutively at c, d, e, &c., until we had reached i, where we should leave him, and again make use of B for the next foresight between i and F, wherever that might happen to be.

Chaining; setting out a Line of Levels.—The duties of the principal chainman, or follower, are as important as those of the staff-holder, for we must depend on him for distances, and therefore as to the correct position of the levels. An error in chaining may vitiate a section as much as an error in levelling; it may cause the total length to be wrong, or a reduced level to be shown in the wrong place. The chainman's duty is to give the distance from staff to staff, and also the distances to the different fences, when the staff is not set up on either side of them, which is occasionally the case; the distances to these fences should then be entered on the blank page of the levelling-book, which, as already observed, is devoted to remarks. It is of importance to do this, as then by means of the plan it is a check on the chainage, as given by the follower. Unless a very safe man, he should never be allowed to proceed beyond the foresight staff, as this gives the observer an opportunity of reading the chainage himself as he passes onwards; the chainman should be particularly careful to call out every ten chains, or change of arrows; neglect of this precaution is sure to lead to fatal errors. In our remarks under the head of surveying, so much has been said about chaining that further remarks are unnecessary; the importance of the subject is evident enough, and where we can. not secure a good chainman we always chain ourselves, employing a man to carry our level as we go along. We may as well mention here that when pressed for time, it may be economised by employing a man to carry the level, and casting out and reducing in the field ; it is only, however, where the surface of the

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LONG SIGHTS AND SHORT SIGHTS,

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ground admits of long sights that much time is saved, and it is only after having had experience, and with good assistants, that where time is very precious, this course is advisable. We prefer ourselves setting up the staff at every mile, and stop the chainage there to recommence for the next mile, and in the column of distances we enter the distance as so many miles instead of so many eighty chains.

The subject of setting out a line of country along which the staff is to be set up, in order to take the levels, now demands a few observations. As a guide over the district, a set of tracings from the plan of the lands through which the works proposed are to be carried, and on which the centre line is laid down, is taken into the field; by means of these tracings it will be evident that the line along which the levels are to be taken may be easily set out; to facilitate this, and to avoid the necessity of scaling in the field, it is customary to mark on the tracings the distances from neighbouring fences, buildings, corners of roads or fields near which the line crosses, always selecting on the plan or tracings the objects nearest to the centre line, as it then gives less trouble to measure off the distances in the field. If at the spots thus measured off "whites” (that is cleft sticks with pieces of paper stuck in them) are set up, the line of levels is set out, except through very large enclosures proposed to be crossed by curves; for these allowances must be made according to the deflection or bend of the curve; and in sidelong ground this must be done with caution in proportion to the steepness of inclination. Of course we are not now alluding to a working section, the centre line for which would be accurately set out before the levels are taken.

We hope that the above observations have sufficiently explained the subject of chaining and setting out a line of levels.

Longsights and Shortsights. This is a subject of some importance if extreme distances are repeatedly taken. If along a line of country having a very gentle rise, many backsights of some twenty chains in length were taken with corresponding foresights of only three or four chains, the correction for curvature would in the long run give a very appreciable difference of level; but, in the first place, these very long sights are very difficult to read off to the hundredth of a foot, even with a very good telescope, and with every pains being given to the adjustment of the eye-piece, and by the milled head to obtain distinct vision and destroy parallax. The reader may satisfy himself of this at any time; a gloomy day, or a very bright sunshiny day make the difficulty all the more palpable; in the second place, the instrument requires for such long sights to be in a perfect state of adjust

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