by 2; or by multiplying the base by half the perpendicular, or the perpendicular by half the base. EXAMPLE. The base AC is 600, and the perpendicular EB 300 links; required the area. The perambulator is used for measuring roads and other level distances; but is unfit to measure uneven ground. It consists of a wheel, the circumference of which is 8 feet, or half a perch; and is furnished with an index, which indicates the distance measured. Various scales are used to protract and measure the plan on paper, such as plane scales, protractors, reducing scales, parallel and perpendicular rulers, &c. The best scales are those used on the Ordnance Survey of Ireland, the Tithe Survey of England, and on the different Railways. They have their edges, which are bevelled off, divided according to the size intended for the map to be drawn to. All distances are measured on the edge of these scales, without the use of compasses. The following table, showing the decimals answering to every rood and perch in the acre, will be found useful to the land surveyor : To Survey with the Chain only. The Parish Survey of England, under the Tithe Commissioners, is in general executed with the chain, which, when used with care and judgment, gives in surveys of moderate size, results sufficiently accurate; but that it should supercede the use of the theodolite, in such an extensive survey as that above alluded to, it is not easy to conjecture. When we use only the chain, we are always limited to one figure, namely, a triangle, the sides of which should be measured with great accuracy, a task extremely difficult, when the ground to be chained is hilly, or covered with clumps, bushes, or other obstacles; such as high walls, thick hedges, deep ditches, &c. A small error in the length of even one of the sides of the triangle will affect the correctness of every part of the survey depending on that side, and also the relative position of the ground when delineated on paper. Hence the necessity of using extreme caution in the admeasurement of the sides. In measuring lines on slopes, a certain number of links must be deducted from each chain, so as to reduce them to the horizontal measurement, otherwise the survey cannot be plotted, and the result will show a greater quantity of land than there really is. For this purpose the theodolite is generally employed. Having levelled the instru ment, set a mark B, at the exact height of the optical axis B A D E of the telescope, at the extreme * The theodolite, with the method of levelling and adjusting it, shall be fully described further on. end D of the slope to be measured, then directing the telescope towards the object, elevate it till the cross wire intersect B, then on one side of the vertical arc will be the angle of elevation of the slope CD, and on the other the number of links to be deducted from each chain of the slope CD, in order to reduce it to the horizontal measurement CE. As an example, let us suppose the angle of elevation to be 16°, then on the other side of the arc will be found the figure 4, which signifies that 4 links are to be deducted from each chain contained in the line CD, to find the horizontal distance CE. Where it is required to take many offsets, or cross several fences, it will be of advantage to make the necessary allowance in the field, noting only the horizontal distance in the field book; but where great accuracy is required, it is best to note the angle of elevation and length of the slope, and make the necessary allowance from the following table, when plotting the work. By referring to the following table for reducing hypothenusal lines to horizontal, it will appear that a reduction of only 3.874 links ought to be made when by the theodolite you make a reduction of 4 links, shewing that where extreme accuracy is required, recourse must be had to the tables. Reduction in Links and Decimals, upon each Chain's length, for the following Vertical Angles. Angles. Reduction. Angles. Reduction. Angles. Reduction. the When properties are separated by hedge and ditch, it is necessary to ascertain from one acquainted with usage of the country, which is the boundary; as in some places the brow of the ditch is the boundary, while in others the roots of the quicks, or the foot of the bank, forms the line of separation. In crossing a ditch or very abrupt precipice, the chain should be kept in a horizontal position, and the end marked by a plumb suspended therefrom. The follower should always hold his arrow close within the handle or loop at his end of the chain, while the leader |