should be connected with longitudinal channels parallel to the formation to gently convey the water to the nearest outlet. The location, distance apart, depth, width, and direction of the trenches must be governed by the nature of the soil, the depth of a cutting or height of an embankment, the area of the surface to be drained, the quantity of water in it, the presence of water-seams and weak places, and by other minor considerations. Footnote 1: _Vide_ Chap. VI., pp. 111, 112. With respect to the material with which the trenches should be filled, a uniform substance having considerable power of absorption, and but few particles between the interstices, is to be preferred, as the trickling of water and vibration causes the smaller material to fall towards the bottom of the trench, which, therefore, may become partly choked, and free drainage be interrupted at the toe of the slope, the most vulnerable place. As sand is always present in unwashed gravel, it will gradually flow or fall to the base of the trench and will prevent equal drainage, but properly burnt clay, being a more uniform and fragmentary substance, is better than sandy gravel, as it is dry and porous; but much depends upon it being well burnt or it may weather. Ashes and chalk are excellent collectors of water and are usually of an even character, but ashes are better than chalk, as the latter material, unless very hard, is liable to become disintegrated by the action of the atmosphere, rain, and frost. The difficulty with respect to ashes is to obtain them in sufficient quantity, free from dust, and of the requisite size. When weight is required as well as drainage, burnt clay, gravel, or chalk is to be preferred to ashes. Burnt clay, although burned upon the site, is generally a more expensive material than gravel, but the amount of moisture in the clay will principally determine the quantity of fuel necessary to burn it, and therefore the cost. If shale is present, by thoroughly igniting it the expense of burning may be nearly reduced to that of lighting and turning over, as it will usually burn unaided. All trenches should drain into pipes placed below the formation or into open drains at a sufficient distance from the toe of a slope as not to deleteriously affect it, in order that the water may be controlled and gently conveyed to an outlet.