The next step is to locate the contour lines of the land, or the lines of equal elevation,—also called the _horizontal lines_,—which serve to show the shape of the surface. To do this, stake off the field into squares of 50 feet, by first running a base line through the center of the greatest length of the field, marking it with stakes at intervals of 50 feet, then stake other lines, also at intervals of 50 feet, perpendicular to the base line, and then note the position of the stakes on the maps; next, by the aid of an engineer’s level and staff, ascertain the height, (above an imaginary plain below the lowest part of the field,) of the surface of the ground at each stake, and note this elevation at its proper point on the map. This gives a plot like Fig. 5. The best instrument with which to take these levels, is the ordinary telescope-level used by railroad engineers, shown in Fig. 6, which has a telescope with cross hairs intersecting each other in the center of the line of sight, and a "bubble" placed exactly parallel to this line. The instrument, fixed on a tripod, and so adjusted that it will turn to any point of the compass without disturbing the position of the bubble, will, (as will its "line of sight,") revolve in a perfectly horizontal plane. It is so placed as to command a view of a considerable stretch of the field, and its height above the imaginary plane is measured, an attendant places next to one of the stakes a levelling rod, (Fig. 7,) which is divided into feet and fractions of a foot, and is furnished with a movable target, so painted that its center point may be plainly seen. The attendant raises and lowers the target, until it comes exactly in the line of sight; its height on the rod denotes the height of the instrument above the level of the ground at that stake, and, as the height of the instrument above the imaginary plane has been reached, by subtracting one elevation from the other, the operator determines the height of the ground at that stake above the imaginary plane,—which is called the "_datum line_." [Illustration: Fig. 4 - MAP OF LAND, WITH SWAMPS, ROCKS, SPRINGS AND TREES. INTENDED TO REPRESENT A FIELD OF TEN ACRES BEFORE DRAINING.] Fig. 4 - MAP OF LAND, WITH SWAMPS, ROCKS, SPRINGS AND TREES. INTENDED TO REPRESENT A FIELD OF TEN ACRES BEFORE DRAINING. [Illustration: Fig. 5 - MAP WITH 50-FOOT SQUARES, AND CONTOUR LINES.] Fig. 5 - MAP WITH 50-FOOT SQUARES, AND CONTOUR LINES. [Illustration: Fig. 5 - MAP WITH 50-FOOT SQUARES, AND CONTOUR LINES.] Fig. 6 - LEVELLING INSTRUMENT.(4) [Illustration: Fig. 7 - LEVELLING ROD.] Fig. 7 - LEVELLING ROD.