Proceed in like manner with the next interval,—3 to 7.
10th. Subtract the numbers in the sixth column from those in the fifth, and the remainders will be the depths to be entered in the last.
Under the head of "Remarks," note any peculiarity of the drain which may require attention in the field.
The main lines A, D, and E, and the drain B, should next be graded on the plan set forth for C, and their laterals, all of which have considerable fall, and being all so steep as not to require silt-basins at any point,—can, by a very simple application of the foregoing principles, be adjusted at the proper depths. In grading the stone and tile drain, (H, I,) it is only necessary to adopt the depth of the last stakes of the laterals, with which it is connected, as it is immaterial in which direction the water flows. The ends of this drain,—from H to the head of the drain C10, and from I to the head of C17,—should, of course, have a decided fall toward the drains.
The laterals which are placed at intervals of 20 feet, over the underground rock on the east side of the field, should be continued at a depth of about 3 feet for nearly their whole length, dropping in a distance of 8 or 10 feet at their lower ends to the top of the tile of the main. The intervals between the lower ends of C7c, C7d, and C7e, being considerably more than 20 feet, the drains may be gradually deepened, throughout their whole length from 3 feet at the upper ends to the depth of the top of the main at the lower ends.
The main drains F and G, being laid in flat land, their[pg 112] outlets being fixed at a depth of 3.50, (the floor of the main outlet,) and it being necessary to have them as deep as possible throughout their entire length, should be graded with great care on the least admissible fall. This, in ordinary agricultural drainage, may be fixed at .25, or 3 inches, per 100 feet. Their laterals should commence with the top of their 1/4 tile even with the top of the 2-1/2 collar of the main,—or .15 higher than the grade of the main,—and rise, at a uniform inclination of .25, to the upper end.
Having now computed the depth at which the tile is to lie, at each stake, and entered it on the map, we are ready to mark these depths on their respective stakes in the field, when the preliminary engineering of the work will be completed.
It has been deemed advisable in this chapter to consider the smallest details of the work of the draining engineer. Those who intend to drain in the best manner will find such details important. Those who propose to do their work less thoroughly, may still be guided by the principles on which they are based. Any person who will take the pains to mature the plans of his work as closely as has been here recommended, will as a consequence commence his operations in the field much more understandingly. The advantage of having everything decided beforehand,—so that the workmen need not be delayed for want of sufficient directions, and of making, on the map, such alterations as would have appeared necessary in the field, thus saving the cost of cutting ditches in the wrong places, will well repay the work of the evenings of a whole winter.