To ascertain that any earth is uniformly affected throughout the mass, and to prevent or provide against deteriorating influences is the chief aim. It is useless to declare any earth possesses considerable cohesion when the power can be quickly dissipated by ordinary atmospheric action and even become a cause of movement, and to rely for permanent stability upon such property. In ordinary earths, not rock, it will generally be found that cohesion is small or insignificant in soil having a coefficient of friction of some moment, and the reverse. In most earths friction, although it is affected in a greater degree by vibration, has to be relied upon, and not cohesion, as the latter is variable and may exist almost unimpaired in a lump, which nevertheless may become detached because of fissures. The coefficients of friction of different earths are also better known than the cohesion; but how easily even friction is impaired may be gathered from the sudden manner in which bridge cylinders will sink after having hung for days by surface friction, or been held by the transitory expansion of clay. Mr. Wilfrid Airy, B.A., in a series of experiments on the cohesion of earth, found that in strong brick loam it is about 168 lbs. per square foot, in compact clay and gravel it may reach 800 lbs. per square foot of section, and in clean sand it is practically nihil. In rock the slope of repose depends whether the earth is unstratified or stratified; and if stratified, upon the dip of the strata and their resistance to the effects of the weather. A vertical face may be stable in unstratified or stratified rock, provided in the latter case it does not dip towards a cutting; or the requisite slope may range from ¼ to 1 to such an inclination that the cohesion and friction, which vary greatly, are sufficient to prevent movement. In sidelong ground should the rock dip parallel to the surface the hill slope may require to be flat, whereas the valley slope may stand vertically. Although, perhaps, in many instances the slopes of cuttings and embankments have been arbitrarily fixed, it may be said, on the whole, no very serious interruptions to traffic have been caused from the _sole want of sufficient initial flatness of a slope_, as that will soon become known. In determining the inclination it is not the angle at which the earth will stand at the time of excavation or deposition, and for a few months after that is required to be ascertained, but that which will _permanently_ suffice to prevent movement. It is well known that almost all freshly cut soil stands nearly vertically for a small depth for a few days in ordinary weather but then begins to crumble and finally break away. What then will be the slope of permanent stability? This chiefly depends upon the degree of exposure, the effects of the weather, water, and vibration upon the soil, and the depth or height of a cutting or embankment.