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96. An arrangement of strata, which is very apt to discover springs rising to the surface, is shown in fig. 41,

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in which the district appears to have a concave surface, with the porous stratum occupying the lower position. The water contained in the higher portions of this bed will burst forth at any outlets that may be formed through the clay; and indeed, if the latter be not of great depth, it will frequently force passages for itself, and thus augment the lower surface accumulations, which are collected at the middle of the section in consequence of its form, and the density of the upper clay. Efficient drainage will, in this case, require that the channels intersect the porous stratum, and, if the depth be not too great, the beds of the drains should reach that of the stratum. The position of the drains on the plan should also be determined, with a view to cut off the water from the gravelly bed at the higher parts of the section, and thus relieve the central main drain at R, which would otherwise become overloaded. Three main drains, therefore, or more, if the sides of the basin be of great extent, should be laid; viz., one at the middle of the section, and two at the higher part, on either side of, and parallel with it.

97. In fig. 42, the strata are represented in positions

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which produce swamps or morasses. Thus, at the point D, at the foot of a porous bed, lying upon one of clay, which rises from that point, the accumulation of water will require a main drain to be laid, bounding the base of the permeable stratum throughout the entire district, and to have a capacity in proportion to the extent of that stratum. If the clay be of inconsiderable thickness, the main drains should intersect it completely. In this arrangement it will be manifestly useless to cut channels above the point D, except as shallow feeders to the mains. This section illustrates one of the reasons of the failure of the methods formerly adopted of attempted drainage without consulting the structural condition of the soil.

98. Sometimes a tongue of gravel, or other pervious material, will be found to extend into and under the clay, as shown in fig. 43, in which a main drain at D, whatever its dimensions may be, will not be sufficient to intercept the drainage water which passes through the bed, and will require another main at Da. In this case, indeed, the principal drain should be laid at this point; otherwise, that portion of the district lying between D and Da will remain in a moist and swampy state.

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99. If, however, the position of the strata be reversed, and the clay runs into and beneath the porous material, as represented in fig. 44, the main drain at D,

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should, if practicable, be cut through the clay, so that the water may be assisted in draining from it, and keeping the space from D to Da in a healthy condition. At the latter point, the depth of the main should be such as to reach the bed of the clay and prevent the water running back towards the point R, in case the inclination has a tendency to produce that effect.

100. A patch of gravel or similar material is occasionally met with in the midst of a district, the surface of which, in other parts, consists of clay, as shown in

fig. 45. In this case two sets of main drains will be

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required, viz., at the points DD, and Da D2; and the same remarks as to the relative depths of these mains will apply as already made in referring to fig. 43. 101. Fig. 46 shows a similar patch of clay running

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into and under the gravel, requiring also two sets of main drains, which will be more effective in proportion to their depth, and the most so if they reach the bed of the clay, and thus prevent its injurious retention of the drainage water from the gravel or sand overlying its edges.

102. When the general surface of the district has a

considerable inclination, as shown in figs. 47 and 48,

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the methods of drainage to be adopted will be varied according to the relative positions of the materials. Thus, if the porous material be above, as in fig. 47, the main drain should be at the point D; but, if the clay lie upon a stratum of less density, as in fig. 48, the main should be laid at a lower situation, where it will naturally receive all the water which accumulates at D, besides that contained in so much of the lower bed as is above it.

103. If a bed of gravel lie in the hollow of a stratum of clay, as represented in fig. 49, the surface of the district will remain tolerably dry except at the lowest point D, where the accumulation of water from the higher parts, resisted in its disposition to descend by the substratum of dense texture, will make a principal main drain of ample dimensions necessary. Auxiliary

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