With regard to the formation width, _i.e._, the width of the bottom of a cutting or the top of an embankment, and the prevention of slips in earthwork, ample breadth is necessary in cuttings for the purposes of drainage, although less in a rock cutting than in that of ordinary soil, and the extent of the top of an embankment has some influence in the promotion of stability. The required width of the formation must be principally regulated by the character of the earth, the amount and suddenness of the rainfall, the height of an embankment or depth of a cutting, the degree of exposure, the exigencies of the traffic, and the required drainage. In wet cuttings the formation should be wider than in dry earth, and the side ditches should be made larger, especially when there is a steep gradient in a long cutting, in order to keep the formation and the permanent way in as dry a state as possible and aid traction, for the coefficient of friction of the wheels of a locomotive will then be greater than when the rails are damp and greasy. In cold climates the width of the formation is often increased so as to lessen the depth of snowdrifts, and to enable a snow-plough or men to more readily deposit the excavated snow and clear the track, as has been referred to in the immediately preceding pages; and in severe climates in wet places ample width in cuttings is found necessary, as drains frequently have to be cut in them as deep as 4 to 5 feet to afford a free flow for and to prevent an accumulation of water. In temperate climates the width can be much reduced. When the formation is narrow the simple percolation of water through the slopes of an embankment, unaided by any aqueous action caused by fissures, may gradually saturate the mass, and as the wider the formation the larger the cross-sectional area of an embankment, it follows that increased resisting power to deterioration is obtained by widening the formation, as there is more earth to absorb the water. A train upon the permanent way will make a force act downwards until it meets with sufficient resistance to cause reaction. The direction of the resolution of these forces is towards a slope, therefore, the farther the surface of a slope is from the line of action and reaction, the greater is its distance from the disturbing force and the lateral resistance it receives from the mass. In the case of a solid rock foundation and a homogeneous embankment in the same state of consolidation throughout its mass, the direction of the forces might be accurately delineated, but as such a uniformly homogeneous and equable condition seldom exists in railway embankments, it cannot be absolutely said that the forces act throughout upon certain lines; however, it is advisable to ascertain the probable direction of the forces.