This instrument is used thus: The fall of the ditch from end to end being ascertained by the spirit-level, and the length also, the fall per rod, or per one hundred feet, may be computed. The span is then placed in the bottom of the drain, from time to time, to guide the workman, or for accurate inspection of the finished cut. We have constructed and used this level, and found it very convenient to test the accuracy of the workmen, who had opened drains in our absence. A ten-foot span will be found as large as can be conveniently carried about the farm. For the accurate grading of the bottom of drains, as the work proceeds, we have in practice found nothing so convenient and accurate as the arrangement which we are about to illustrate. The object is simply to draw a line parallel with the proposed bottom of the drain, for the laborers to work under, so that they, as they proceed, may measure down from it, as a guide to depth. Having with the spirit-level, ascertained the fall from end to end of the drain, a short stake is set at each end, and a line is drawn from one to the other at the requisite height, and supported by the cross-pieces, at suitable distances, to prevent the sagging of the line. [Illustration: Fig. 59.--GRADING TRENCHES BY LINES.] Suppose the drain to be ten rods long, and that it is intended to cut it four feet deep, the natural fall being, from end to end, sufficient. We drive a stake at each end of the drain, high enough to attach to it a line three feet above the surface, which will be seven feet above the bottom of the finished drain--high enough to be above the heads of the cutters, when standing near the bottom. Before drawing the line, the drain may be nearly completed. Then drive the intermediate stakes, with the projecting arms, which we will call squares, on one side of the drain, carefully sighting from one end of the stake to the other, at the point fixed for the line, and driving the squares till they are exactly even. Then attach a strong small cord, not larger than a chalk line, to one of the stakes, and draw it as tight as it will bear, and secure it at the other stake. The line is now directly over the middle of the drain, seven feet from the bottom. Give the cutters, then, a rod seven feet long, and let them cut just deep enough for the rod to stand on the bottom and touch the line. Practically, this has been found by the author, the most accurate and satisfactory method of bringing drains to a regular grade.