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hills and valleys, cuttings and embankments, tunnels and viaducts, &c.; as regards materials, where in the first case we should look for a gravelly district, in the latter we should seek for clay, It is more particularly as regards water for feeding the canal, and the position of locks and chains of locks, and length of ponds, that the survey for a canal will differ from that for a railway.
Setting aside the question of traffic in a canal survey, for which, however, it must generally consist largely of minerals, and particularly coal, besides agricultural produce, to be of any value to shareholders, the engineer will have to examine for the supply of water more particularly at the summit levels; where it is to come from and how far brooks and streams are appropriated and to what purposes, as for mills, irrigation, &c.; what is to be the connexion with any neighbouring river or canal, if any; the length of long level ponds to be obtained, thereby saving the trouble and loss of time attendant upon locks; by this means a number of locks are often brought together, so that they can be attended to by one lock-keeper; the trial sections will enable bim to judge of the position of long levels to be obtained, of the lowest summits to be found between the termini ; whether a summit should be reached by a cutting or a tunnel, and also to decide as to other works involved by adopting the whole or part of any particular line, the features of which are shown on a trial section. If after adopting a summit level, and calculating the obtainable quantity of water, with its cost by compensation, if any, or otherwise, or by pumping if necessary, it may be found desirable to trace the level of the summit along the sides of the range of hills, which will involve a series of contour levels,* and at the same time a survey of every spring and stream crossing
a this level line ;t care is to be taken all along the line to distinguish landsprings, or such as only run in winter. The line of contour level will have to be laid down on a plan, on which must also be shown every hill and valley above this level and draining towards it, as also the water ridges, in order that from such data the area of drainage may be calculated above the contour level; from this, if it should prove desirable, may be calculated the capacity required for reservoirs, as well as the quantities of water to be expected to flow into them; the best information attainable must be sought for with regard to the depth of rain-fall, and numerous rain-gauges must be established at different points to corroborate or correct the information obtained. Not only all
* See “ Contour Levelling.”
+ All the streams running into or crossing the proposed canal will require gauging, and these gauges will require to be established both above and below the level of the crossing of the canal. See “ Gauging Streams.”
information obtained, and the observations made, must be entered in a book kept for the purpose, but they must be recorded with all details connected with them in such manner that they may be distinctly and accurately referred to hereafter.
Certain points being fixed by the levels of the trial section, it will often happen that long level reaches may be established by tracing a contour level along the ground, as, for instance, by the side of a valley until such fall occurs as renders necessary one, two, or three locks, or a fall of from 20 to 30 feet; the depth of this fall being transferred, the level thus found may again be traced forward until the same operation is repeated. As these levels are thus traced a stake may be driven where the levelling staff is held, so that a surveyor coming after may make a survey of them and lay down the lines on the plan; with the assistance afforded by the levels on the trial sections, all referring to one datum, a new and improved general direction may be given to the line, along which another section may be afterwards taken, such deviations being made as seem advisable under contingent circumstances.
In tracing these levels it will be remembered that for canals we are not tied down to curves of large radius, as in railways, and that we have here considerable latitude, it being only requisite absolutely that the boats may be conveniently worked.
Provision will also have to be made for weirs and overfalls, and generally these should be constructed where the floods occur; it will, therefore, be necessary to examine into this subject also.
Such observations being made, and levels traced and laid down on the plan, it will be time to revise the whole scheme, and see what bends may be avoided by cuttings or embankments, as by the plan above-mentioned of tracing contour levels it will be perceived that in many cases the line would be very crooked, and the bends must be overcome.
The above contingencies connected with the supply of water make the circumstances of an exploring survey for a canal considerably different from that for a railway, although the works from point to point are the same, such as cuttings and embankments
, tunnels and viaducts, bridges, &c., with the exception, however, of the way itself.
In both we have to reach from one place to another, supplying directly or indirectly certain towns and villages on our way, and we have to join the two ends by the shortest and most level route compatible with economy, safety, and rapidity of transit, &c.; generally there are certain summits or lowest points to be overcome, and it will be a matter of study whether these are to
be passed by tunnels or viaducts with easy gradients more or less all the way, or whether a steeper inclination shall be assumed with a partial deviation in route, so as to have merely a cutting or embankment where a tunnel or viaduct was first assumed; sometimes the route by such a summit or low point may be abandoned by means of a lateral deviation of more or less extent, as may be required; the crossing of a river or a town, or even a turnpike road, will probably considerably affect the study of a scheme. As regards railways, there are cases where a little study will enable an engineer to take the surface pretty nearly the whole way, with merely a few feet of cutting and embankment here and there; where the roads to be crossed are not too numerous or too important for level crossings, this is of course an economical system.
Working Plans.--Details of Townlands' Surveys.—Precau
tions to be observed.-Traversing.–Object and Value of Working Plans.-Concluding Observations on Land Surveying.
THE observations made in the last chapter apply principally to preliminary surveys, such as are prepared before a working or " permanent” survey is made previous to the actual construction of the works; as already observed, a certain width, being a few chains more or less, is adopted; but for the working survey, for which we are confined for the works at least within the “limits of deviation” assigned under powers obtained from an Act of Parliament or otherwise, it is a sheer waste of time and money to extend the survey beyond such limits; otherwise than this the same system is to be adopted for the working survey as for the preliminary one, with the exception principally that the offsets are shorter and more numerous, and a larger scale adopted for the plan ; the same lines may often be taken as bases, but less offsetting will fall on them, as the actual survey is now restricted to the limits of deviation. Another plan to adopt with regard to a working survey is to “stump out the line” first, as soon as final alterations are determined on, and then to make a survey of the line thus stumped out, extending the width of it from 30 or 40 feet up to 100 feet on each side, according as such greater width may be required by the extent of the works, such as the slopes of cuttings and embankments or the inclination of approachroads.
The above remarks apply equally to the working survey of townlands; here, however, a traverse is almost the only mode to be adopted, the sides of the traverse running down the sides of the street more or less, and where these are wide there being a chain line on each side; on the accuracy of such surveys often depends the purchase or non-purchase of certain property, and the greatest care must be devoted to the work; the offsets should not exceed 20 or 30 feet; in fixing the positions of buildings it is
not sufficient to do so by merely taking an offset at right angles to the chain; but two and sometimes three measurements should be taken from different links on the chain to one angle on the buildings, thus, in fact, setting up two or three small
triangles, the sides of which are checks on each other. The offsets off the chain lines A B and CD, Fig. 79, will express this as well as a more lengthened explanation. At the same time always measure with the tape all the sides of a building, as it will prove a further check upon the chaining and offsets, for unless this has been correctly done the side measurements of the buildings will not plot.
To every side of a building of any length always take three or four offsets, as they will mutually check each other; for the straight side of every building must pass through all the punctures made if all the offsets are correctly taken.
It will often occur, more particularly in old-fashioned places, that some buildings are so irregularly dovetailed into others that plans of the interiors must be surveyed before it is possible to make a plan of the buildings. An instance of this kind is shown on the right hand of Fig. 79, as also the lines that were measured in order to take the plan. In these cases measurements must be taken of the thicknesses of the walls, as the angles formed by the faces are the points measured to. Instances of this kind sometimes arise which are not difficult, but very troublesome. Impatience on the surveyor's part is then what he has most to fear; such matters will take their time to be properly done ; he will feel far more annoyed and mortified if, at a future period, it proves necessary to take down a building which his plan showed would not be touched by the construction of the works; for it must be remembered that to take even three inches off a building it must be purchased, if the owner thinks proper, as in such cases he is pretty certain to do. If the error is only discovered after other buildings from that particular owner have been purchased, it may result in a troublesome business. It will thus be seen that townlands' surveying may afford many instances where it is scarcely possible to observe too much care and caution with regard to the details, as well as with regard to the whole.
It is very good practice to ascertain the exact chainage at which a side produced of any building will intersect on the chain line, and to measure the length of such produced line.
It may be observed that to do all this the sketching in the field-book will have to be drawn to a large size, and the writing and figures to be kept small, and written with a hard and sharppointed pencil.
Another troublesome case often arises where many of the