With respect to the chief organ of stability in earths other than rock, namely, the frictional resistance; friction during motion is generally considered to be less than the force necessary to overcome it when at rest, and undoubtedly this is the case when the surfaces are similar, and are smooth and hard and not easily impressed, as iron, granite, concrete, and metals generally; but when they are comparatively soft and incapable of resisting indentation at any pressure that they may have to bear, the difference between the coefficient of friction during motion and that at the commencement of motion or of repose will not be so marked, for other resistances may come into action not due _solely_ to surface friction of the mass. A surface may become indented or roughened thus offering opposition to motion not existing at the commencement of movement, and particularly so in any earth of a mixed character possessing hard particles, such as boulders, or sand in clay. On the other hand, in the case of hard rock, solid clay, or other homogeneous earth, the difference between friction during motion and that of friction at rest may be reliably determined. In soils of a granular or gritty nature small particles become detached during motion, and by pressure occupy or become wedged into any cavities upon the surfaces, and therefore offer resistance which is not _alone_ due to friction of a mass upon a like mass. From this cause friction during motion may seemingly even become greater than during rest, but with material consisting of rounded particles that will not wedge, the friction upon a sliding surface may be lessened by reason of the grains revolving. In deducing a slope of repose for earth, the lowest value of frictional resistance, whether during motion or at rest, should be taken, and always as if the surfaces were wet. The coefficients of friction, F, during motion usually range between 0·25 to 1·10, and the slope of repose, S to 1, is consequently found by the expression— S = 1/F, therefore, S would equal to 1/0·25 to 1/1·10 = 4 to 1 TO 0·91 to 1, say, 1 to 1.