ponding to the length of the standard chain, notches being cut, or nails driven to mark its exact length. The coping of a horizontal wall is also well suited to receive such marks. They are thus rendered permanent, and every day a comparison of the working chain may be made with the standard length without loss of time. This precaution is indispensable; for the chain, being composed of numerous pliable links joined together by three small unwelded rings which give it flexibility, is from its construction constantly liable on the one hand to expand at the joints, and on the other to have the links bent when dragged over rough surfaces. Of such importance is this examination held by scientific and practical men that the Commissioners for the restoration of the Standards of Weight and Measure recommend in their report, dated December 21, 1841, that "no person shall be admitted to give evidence in any court of justice of having measured land, after the passing of the contemplated Act, with any other than a stamped (standard) measure, or a measure which has been compared, on each day on which any part of the measurement has been made, with a stamped (standard) measure." The chain used for measuring may be left unaltered, if it be about half an inch longer than the standard, as it can never be stretched perfectly straight, but adapts itself by its weight to the small inequalities on the surface. The chain also, when used in wet weather, becomes shorter, in consequence of the insertion of dirt between the rings. If the excess be great, the length must be diminished by the removal from each extremity of one or more of the rings. The same correction, whatever it be, should be made equally at both ends, in order that the middle point of the chain may be in its true position, in which case the error, subdivided among the remaining parts, will be trifling, and not worth being taken into account. If, on the contrary, the working chain be found too short, this will arise from the links being bent; consequently the correction will be effected by straightening them. Surface to be divided into Geometrical Figures. In measuring an estate, a parish, or any comparatively small portion of land, the surface may be supposed to be divided into a system of arbritary geometrical figures bounded by right lines, either inscribed within or circumscribing the area to be measured; but so disposed that they shall pass near all objects included in the survey, and serve to determine their positions and forms. The object of this imaginary division is to facilitate the measurement of all irregular boundaries, which, if they were traced independently of these auxiliary lines, into all their windings, would lead to a great consumption of time in the operation and to inaccuracy in the results, owing to the number of mutually dependant angles to be measured. After having divided the surface into a number of geometrical figures bounded by right lines, the sides of these figures are used as bases from which the irregular boundaries and other objects are measured by means of shorter lines at right angles, termed offsets. Offsets. These offsets, when short, are measured with an offsetstaff 10 feet in length; and with a second chain, or, in preference, a measuring tape, when the offsets are too long for the staff to measure them conveniently. The limit to the length of these offets is fixed in a great measure by the degree of accuracy aimed at in the survey, as well as by the scale to which the plan is to be drawn; in general it is not advisable to make use of offsets more than about 100 links in length. Offsets, with few exceptions, are measured at right angles to the main line: the length of the offset being determined, the position of the object referred to is fixed with reference to the main line. These right angles must of course be set off correctly. When the length of the offset is only a few links, the right angle is generally set off by the eye. visable to use an instrument. For long offsets, it is ad- for setting out perpendicular lines was the cross-staff. The Cross-Staff or Surveying Cross. The simplest form of the cross-staff consists in a cubical top formed of hard and well-seasoned wood, fixed on a pointed staff nearly equal in height to the observer's eye, to prevent stooping. Two vertical cuts are made by a tenon-saw, exactly at right angles to each other, of such a depth as to leave enough wood to maintain the parts firmly in their places. The saw-cuts form the "sights." But, to extend the field of vision laterally, two centre-bit holes should be made at the bottom of the cuts, to be used in the first instance in finding the objects. This instrument can evidently be made with ease by any carpenter. A surveying cross (see figure page 8) less simple, and more easily put out of adjustment, consists of four upright plates (usually of brass) called "sights," fixed at right angles on a cross-shaped piece of metal having a socket underneath to receive the staff. Either kind of instrument being thrust into the ground with two of the sights placed in the direction of the main Line Sight line, points out the perpendicular required, by means of the other two sights. This instrument presents the advantage of simplicity; but it cannot be used on hard roads or in towns; and in soft or swampy ground it is not to be relied on. Hence it has been generally superseded by the optical square, or reflecting surveying cross, a small circular box, of about 2 to 3 inches in diameter, which marks a right angle with accuracy and expedition. Optical Square. This box contains a strip of looking glass, from the upper half of which the silvering is removed so as to admit of direct vision through it, while the lower half acts as a reflector. The eye placed at E, will observe the dis tant object B by direct vision through the hole b and upper unsilvered half of the reflector; while, if another object A is placed at right angles to the line E B, it will be seen through the hole a, and reflected by the lower silvered half of the mirror to the eye at E, and thus appear to coincide with the object B. The direction of the half-silvered glass is adjusted so as to form half a right angle, or 45°, with the line E b. With this instrument, therefore, a line may be laid out at right angles to another, from any point in it, by simply standing over the given point, and looking through the eye-slit along the line, having an assistant to go with a mark or ranging rod in the direction in which the perpendicular is required, and signing to him to move to the right or to the left, until his rod is seen by reflection to coincide with a staff fixed on the line along which the observer is looking. When the coincidence takes place, the rod is fixed in the ground. If, instead of erecting a perpendicular from a given line at a given point, it be required to find on a line the point of intersection of a perpendicular from a fixed object, as a house, a tree, &c., the observer himself must move along the line until the image of the object appears, as before, in the direction of the line, and the place where he then stands marks the spot where the perpendicular would fall. Method of raising a Perpendicular with the Chain only. The following is a correct method of setting out a perpendicular with the chain only. Let ab be the line on which it is required to erect a perpendicular from the point a. Fix an arrow in the ground at a, through the ring of the chain denoting 20 links, and measure 40 links on a b. At b fix the extreme end of the chain; then, holding |