form having for object the determination of the relative position of any single distant point; but it will be evident that the same principles apply to the practical determination of position of any number of points, which is the object of surveying operations. A skeleton groundwork of points, distances, angles, &c., is the substratum of the final delineation of roads, hedgerows, rivers, buildings, quarries, bridges, tunnels, rocks, shoals, outcrops, capes, and harbours, &c., that may be required; and though this final representation should be so far correct that no appreciable error should be possible, and nothing omitted that is essential to the object of the survey, yet the choice of modes of procedure, as of instruments to be used in the skeleton-work, is so much at the pleasure of the surveyor that abstract rules for guidance are perfectly impossible. The special objects of the survey, the scale on which the plan is required, the amount of time, number of men, and the sort of instruments available, are the matters that guide the surveyor in laying down the general arrangements; while, again, methods and details. of execution are best left to the discretion of those competent to be entrusted with the work, who are then more capable of producing good results. Verification or checking.-There is, however, one principle or rule that may be considered an established maxim in all surveying, which is absolute under all circumstances. It is that some means of verification be forthcoming for every part of the work done, or that nothing be made entirely dependent on a single measurement of distance, or a single angular or other observation of any kind, in which an accidental mistake may occur without possibility of discovery, and thus vitiate results. The element of uncertainty must be completely eliminated. If a certain number or amount of measurements and observations are just barely sufficient to enable the series of required points to be arrived at and delineated, as, for instance, in a single triangle, either two adjacent sides and the included angle, or one side and the two angles at its extremities are barely enough; then some additional measurement or observation becomes necessary for verification, which in this case would be effected by measuring an additional angle or side. The same principle applies to the extremities of a series of triangles or any extended skeleton work, where either a single verification or a series of checks become necessary. The only case in which checks are dispensed with is in short rectangular offsets to hedgerows and similar minor features, when the memory, or recollection of form and disposition is most generally entirely relied on for verification of result, but in some cases an extra oblique offset to the same point is absolutely necessary as a check. The various sorts of survey-work for general purposes being thus practically very much dependent on the sort of instruments chosen and used, they will be classified and treated in accordance with them in the following sections; while the surveys for special purposes will have their distinctive points subsequently mentioned. The general classification will be as follows: Surveys without instruments for angular measure ment.. I. Chain surveys. 2. Telemeter surveys. Surveys with small instruments for angular measure ment. 4. Compass surveys. 5. Box-sextant surveys. Surveys with larger instruments for angular measure ment. 6, Theodolite surveys over small areas, or over large areas without much intricacy of detail. 7. Theodolite surveys over large areas, or over small areas with much intricacy of detail. Limit of admissible error. Whatever instruments be used, the object is to obtain a correct plan with the least amount of labour. The limit to the necessary accuracy or exactitude in surveying can be directly determined with reference to the scale of the plan, for the plan should have no visible error. Now any distance represented on a plan by a hundredth part of an inch is just fairly appreciable to the naked eye, and if we consider thatth of an inch is inappreciable, this amount in conjunction with the scale of the plan affords a limit to the exactitude required in actual measurement in the field, or extreme bounds admissible in any cumulative error or total discrepancy. For instance, if the survey has for its object a correct plan to the scale of 60 feet to an inch, the limit 0 th of an inch on the plan will represent 6th of a foot or about 3 inches on the ground; if the scale to be used is one mile to six inches, thenth of an inch on the plan will represent about 4'4 feet on the ground. 0 The total error thus admissible on the ground should invariably be thus determined before commencing survey operations, as otherwise even a most careful survey may be either too inexact or too laborious in detail, in the one case causing a total waste of time and labour, and |