Page images
PDF
EPUB

The shoulders, shown in fig. 55, as being left in the soil, may be in some cases better substituted by clearing the drain out level, and introducing two slabs of stone, meeting at the bottom, and having a stone wedged in between them at the top, the upper portion being filled in with loose stones. If the flat stones at the sides can be obtained 6 or 8 in. broad, and 1 in. thick, at about 4d. per ton, an acre of land may be drained with drains 32 in. deep below the crowns of the gathered-up ridges, and laid at distances of 15 ft., at a cost of about 21. 12s., exclusive of carriage of materials, and the ploughing by which the upper ridges are formed. Constructed as in fig. 55, if the covering flags can be procured 12 in. wide, and 2 in. thick, at the same rate per ton, the acre may be similarly drained for 8s. or 9s. less.

136. For the draining of bogs, the arrangement represented in fig. 57 is peculiarly applicable, all the materials being on the spot, as the whole drain is refilled with the peat itself, which is well known to resist the action of water with impunity. Provided the cutting of the drains be done in summer, when the material quickly becomes dried, and sufficient time is allowed between the successive operations of cutting and refilling to effect the required consolidation, no better method is yet devised for effecting this kind of work. The liability of the moss in the bed of the drain to subside in detached parts, and with great irregularity, is certain to destroy any arrangement of tiles, pipes, or other artificial materials.

137. Another mode of forming drains with the natural materials, called plug draining, has yet to be mentioned, rather to make our list complete, than for any extended applicability of which it is susceptible. Plug drains are practicable only in subsoils of very firm clay

entirely free from stones, and which never become thoroughly dry except by evaporation. Its use is further limited to lands, the occupation of which is that of permanent pasture. The usual form of open channel being formed in the clay, wooden blocks are introduced, which fit the lower part of the channel to a height of 8 or 10 in., and are convex or arched on the top. Upon these blocks, or suters, or plugs, the clay is returned and rammed down in a very careful and thorough manner. The plugs are then withdrawn, or drawn forward, for the formation of another length, and leave an open space or duct below for the passage of the water. In Gloucestershire this kind of drain, 2 ft. deep, has been executed at from 4d. to 7 d. the rood of 6 yards, but the whole of the operations have to be conducted with extreme care, when the soil is free from frost or much wet, the ramming, moreover, requiring stout labourers, while much of pipe-draining may be performed by women and children, and the finished work is, after all, peculiarly liable, like all other drains formed with natural materials, to the destructive attacks of moles and other underground vermin.

138. These being the principal forms and constructions of land-drains, some notice of the successive operations to be carried on, and the implements to be employed, is required to complete our account of the draining of districts and lands.

139. The preliminary survey of the district to be drained having been made, and such indications of the nature of the soil as this survey affords noted, the next desideratum is precise information as to the relative level or altitude, from a fixed datum, of every portion of the surface to be drained. These levels are obtained by the spirit-level in the usual manner, but they should

be so complete as will enable us to lay down a plan of the district, with the levels of equal altitude marked upon it. These levels will appear as lines upon the map. Thus the highest point will be represented by a dot. The space around it, at one degree of altitude less, will appear as a continuous line encircling the dot. The space which has one degree of altitude less than this, is represented by another encircling line around the preceding, and so on down to the lowest altitude. These encircling lines, called contour lines of equal altitude, will, of course, be more or less irregular, according to the superficial character of the district. They were used in the French Survey in 1818, having been suggested in an Essay read before the French Academy of Sciences so early as 1742, and since adopted for the Irish Survey in the year 1838. A precise idea of these lines may be formed, by supposing a block of stone, of an irregular conical form, to be immersed in a vessel of water. If the water be drawn off so as to lower its level equally, say one inch, at each successive discharge, the lines on which the water meets the surface of the cone will be the true contour lines of equal altitude. The degree of altitude to be adopted for each contour line will, of course, vary in amount, according to the prominence of the hills and the exactness required, being less in proportion as the district is flat or slightly undulating. From a map thus plotted, combining, as it does, a true plan and infinite sections of the district, any vertical section can at any time be accurately prepared without further reference to the ground, while an immense advantage is obtained in the true indication of all brooks, and outcroppings of the substrata of the soil.

140. The levels being ascertained and recorded, the

observations necessary to complete the information upon which the general arrangement of the drains can be determined, have next to be made by means of boring into and examining the structure of the subsoil at various points of the district. The tools, or boringirons, employed for this purpose, consist principally of the auger, by which cylindrical holes are made in the soil, and their contents brought up to the surface; the punch, with which compact gravel and soft rock are perforated ready for the action of the auger; and the chisel, or jumper, with which, by the aid of sledgehammers, the necessary holes are made in hard rocks which resist the auger and chisel. The auger is from 2 to 4 in. in diameter, and about 15 in. long in the shell. Its form is that of a cylinder, with a longitudinal opening throughout its length, and a sharp cutting edge or nose of steel secured to the entering end of it. The form of the punch is that of a pyramid of four sides, from 8 to 12 in. in length, and from 2 to 4 in. square on the base, which is the upper end of the tool when in use. The chisel is a flat tool, with a broad and cutting end, and is made of various sizes, according to the size of hole required. Each of these tools has a screw formed at the upper end, which will fit rods of iron made about 3 ft. long and 1 or 11⁄2 in. square, used to lengthen the apparatus when the hole becomes deeper than the length of the cutting tool. A cross handle of wood, fitted to a tapped socket that fits upon the tools and rods, is used for the purpose of turning the augers.

141. When the nature of the subsoil has been ascertained by boring, and the arrangement of main and minor drains determined, the work of forming the drains is commenced by marking the lines upon the surface with poles, and driving stakes to indicate the intended position

and direction of the cuttings. A common garden line is then used to mark off the sides of the cuttings according to the width they are intended to have. The ground is then rutted with the spade along the line, and a rut is made at each side of the drain. From 20 to 30 yards are thus lined out at each stage. The different tools to be then employed will vary according to the structure of the subsoil to be removed.

142. For ordinary soils, such as clays, loams, and small gravel, and the usual combinations of these materials, the common spade, the ditcher's shovel, and foot and hand-picks, are the tools employed. The foot-pick has a cross handle of wood, and a tramp fixed at about 15 in. from the point of the tool, on which the foot of the workman is placed to drive it into the soil, and which forms a fulcrum on which stones or lumps of the hard dry clay are raised. The ditcher's shovel resembles a common spade, but is somewhat stronger, and rounded off on each side from the haft, forming a rounded point at the lower end. If the sides of the drain have a tendency to fall in before the work can be completed, they must be sustained by planks and short struts of timber wedged in between them. In order to test the correct dimensions of the drain, a gauge is useful, which consists of an upright stem, on which two or three crosspieces are fitted to move up and down. The stem being graduated, these cross-pieces may be shifted and fixed so as to correspond with the intended limits of the cutting, and applied wherever thought necessary.

143. The uniformity of the fall of drains is tested with these staves, each consisting of an upright stem, and a cross-piece on the top fixed at right angles, so that the form of the instrument resembles that of the letter T. Two of these staves are made about 2 ft. in

« PreviousContinue »