In the case of all earths that are readily impaired by water, it is the degree of permanent uniform dryness or wetness that governs the safe height of an embankment; for even the surface of the most impervious clay will be liable to become in a muddy state, although in the mass it may be dry or in a condition conducive to stability. Almost all aluminous or calcareous earths hold or retain water in varying quantity, and as in embankments this may constantly change, no safe permanent height of an embankment in such soils can be established. The most solid impermeable clay when separated into small pieces and impregnated with water to any quantity above that it normally contains, will have its bearing power and general stability reduced, until, when saturated, it becomes as mud. Impure and argillaceous chalk, marly clay, marl, and shale, are all more or less weakened by aqueous action, and their safe permanent height may be anything not exceeding the crushing strength of the earth, with a due allowance for the deteriorating influences of weather, vibration, and internal water-pressure. In this chapter reference has been made to certain limits of the safe height of embankments; but were these to be adopted as _the_ maximum safe height in each earth it would be an indefensible edict, for by the adoption of the required precautionary measures named in the several chapters of this book the height can be considerably increased, and what is more, has been much exceeded with impunity in numerous instances. The inclination that any earth assumes, after being exposed to the weather a sufficient time for it to be affected by it, will afford a good indication of the safe load it will support. As the slope of repose becomes flatter, the bearing power of the soil in an embankment will be reduced, all the deteriorating influences being identical. For the best information “On the actual lateral pressure of earthwork,” _vide_ the paper by Sir B. Baker, M. Council Inst. C.E., Minutes of Proceedings of the Institution of Civil Engineers, London, vol. lxv., part iii. CHAPTER VI. SLOPES, GENERAL CONSIDERATIONS.—TABLE SHOWING THE USUAL RANGE OF SLOPES.—TABLE OF COEFFICIENTS OF FRICTION.—NOTES ON THE COHESION OF EARTH.—FORM OF A SLOPE.—SOME CONDITIONS GOVERNING THE NECESSARY INCLINATION.—WIDENING EARTHWORKS WITHIN THE ORIGINAL FENCES.