Imágenes de páginas

proper superintendence, to say nothing of the imperfect manner in which the work is executed. A common impression seems to prevail, that if a 2-inch pipe is good, a 3-inch pipe must be better, and that, generally, if draining is worth doing at all, it is worth overdoing; while the great importance of having perfectly fitting connections is not readily perceived. The general result is, that most of the tile-draining in this country has been too expensive for economy, and too careless for lasting efficiency.

It is proposed to give, in this chapter, as complete a description of the preliminary engineering of draining as can be concentrated within a few pages, and a hope is entertained, that it will, at least, convey an idea of the importance of giving a full measure of thought and ingenuity to the maturing of the plan, before tb«1 execution of the work is commenced. "Farming upon paper" has never been held in high repute, but draining upon paper is less a subject for objection. With a good map of the farm, showing the comparative levels of outlet, hill, dale, and plain, and the sizes and boundaries of the different inclosures, a profitable winter may be passed,—with pencil and rubber,—in deciding on a plan which will do the required work with the least possible length of drain, and which will require the least possible extra deep cutting; and in so arranging the main drains as to require the smallest possible amount of the larger and more costly pipes; or, if only a part of the farm is to be drained during the coming season, in so arranging the work that it will dovetail nicely with future operations. A mistake in actual work is costly, and, (being buried under the ground,) is not easily detected, while errors in drawing upon paper are always obvious, and are remedied without cost.

For the purpose of illustrating the various processes connected with the laying out of a system of drainage, the mode of operating on a field of ten acres will be detailed, in connection with a series of diagrams showing the progress of the work.

A Nap of the Land is first made, from a careful surrey. This should be plotted to a scale of 50 or 100 feet to the inch,* and should exhibit the location of obsta cles which may interfere with the regularity of th drains,—such as large trees, rocks, etc., and the existing swamps, water courses, springs, and open drains. (Fig. 4.)

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 staflf, 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 .ine. 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

*The maps in this book are, for convenience, drawn to • scale of W feet to the inch.

[ocr errors]
[ocr errors][graphic]



[graphic][table][merged small]

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"

The next operation is to trace, on the plan, lines following the same level, wherever the land is of the proper height for its surface to meet them. For the purpose of illustrating this operation, lines at intervals of elevation of

* The instrument from which this cut was taken, (as also Fig. 7,) wai made by Messrs. Blunt & Nichols, Water-st, N. Y, 1867.

« AnteriorContinuar »