sary. The distances, which are necessarily short, are measured horizontally with a good tape or chain, and in order to proceed with rapidity, each reading on the staff should be taken nearly at its extreme length and if possible near the top of the staff. The rise or fall at each sight, according as the work proceeds uphill or downhill, is equal to the staff-reading less the height of the level, measured on the supportingrod. (See figure 24). In this work, going uphill is the better plan in practice. Let A be the starting-point of known or assumed level, where the graduated staff is held the observer proceeds uphill in the required direction, and sets his supporting-rod at B, hangs the reflecting-level on it, and takes a reading on the staff at A; the staff-holder then brings the staff to B, and the observer goes on to C to take a reading on the staff at B, and so on. The records are thus kept: Permanent height of level on rod 3.5 feet 51.80-14:00=37.80 feet. Total rise in 145 feet When hill-sides are excessively precipitous, and often exceed an inclination of 30°, the reflecting-level may be superseded by a mason's level, the horizontal distances being measured with a 10-foot staff held to level, and the vertical heights with a graduated level-staff; but when the fall is nearer 30°, the former method is more rapid, and preferable. It is generally more convenient to adhere to straight lines, and not to vary the direction, in levelling on steep hill-sides. Contouring. The difference of level between contour lines to be shown on a plan is made dependent on the scale of the plan, the steepness of the ground, and the object specially under consideration; it may be either one foot or fifty, or any convenient number of feet between the two. The principal contours, which occur at every fourth or fifth contour-line, are surveyed and plotted with special accuracy; the remainder are interpolated in a rougher way. Thus, if the principal contours have a difference of level of 100 feet, the intermediate contours may be taken at differences of 25 or 20 feet; or if the former are taken at every 8 feet of elevation, the latter may be taken at every 2 feet or at each foot. The principal contours are based on a series of sections taken on the principal watercourse and watershed lines of the country, according to which the position of points of any required set of altitudes, such as 100, 200, 300, 400, etc. feet, can be plotted on the plan and marked on the ground; such points are then extremely distant points on the principal contours. Each principal contour is then separately set-out between those points with the levelling instrument, and the points thus set-out are surveyed (generally with compass and chain or telemeter) and plotted on the plan as well as marked on the ground. Intermediate contour lines are interpolated in various ways generally intermediate level-points are set-out and surveyed on every watershed and watercourse line, between which these contours can be either set-out and surveyed, or accurately sketched. In the former case the interpolation of contours should in flat country be done with the water-level, and in steep country with the reflecting-level, whenever rapidity is an im |