A staff of ten feet in length, graduated in feet and inches, and held by an attendant at the various points of observation, is necessary in the use of the spirit-level in the field. A painted target, arranged with a slide to be moved up and down on this staff, and held by a thumbscrew, will be found useful. We have made for our own use a level like the above, and find it sufficiently accurate for drainage purposes. Small spirit-levels set in iron can be had at the hardware shops for twenty cents each, and can be readily attached to wood by a screw, in constructing our implement; or a spirit-level set in mahogany, of suitable size, may be procured for a half dollar, and any person, handy with tools, can do the rest. The sights should be arranged both ways, with a slit cut with a chisel through the brass or tin, and an oblong opening at each end. The eye is placed at the slit, and sight is taken by a hair or fine thread, drawn across the opening at the other end. Then, by changing ends, and sighting through the other end at a given object, any error in the instrument may be detected. The hair or thread may be held in place by a little wax, and moved up or down till it is carefully adjusted. The instrument should turn upon the staff in all directions, so that the level of a whole field, so far as it is within range, may be taken from one position. [Illustration: Fig. 57. STAFF AND TARGET.] To maintain a uniform grade in the bottom of a drain so as to economize the fall, and distribute it equally through the whole length, several different instruments and means may be adopted. The first which we will figure, is what is called the Span, or A Level. Such a level may be easily constructed of common inch-board. If it be desired to note the fall in feet, the span may conveniently be ten feet. If a notation in rods be preferred, the span should be a rod, or half rod long. The two feet being placed on a floor, and ascertained to be perfectly level by a spirit-level, the plumb-line will hang in the centre, where a distinct mark should be made on the cross-bar. Then place a block of wood, exactly an inch thick, under one leg, and mark the place where the line crosses the bar. Put another block an inch thick under the same leg, and again mark where the line crosses the bar, and so on as far as is thought necessary. Then put the blocks under the other leg in the same manner, and mark the cross-bar. If the span be ten feet, the plumb-line will indicate upon the bar, by the mark which it crosses, the rise or fall in inches, in ten feet. If the span be a rod, the line will indicate the number of inches per rod of the rise or fall. [Illustration: Fig. 58.--SPAN, OR A LEVEL.]