2 to 1 TO 3 to 1 is the most general slope; but in marsh land, unless the surface is protected, they may not permanently stand at 5 to 1. Fascine-work may be the easiest and most convenient protection, or a gravel or turf-covering may suffice, and should new channels have to be created, the question to decide is, will it be cheaper to make the slopes sufficiently flat so that they can be left unprotected, or is it preferable to have a steep inclination, such as 1½ to 1, and to carefully cover and protect the surfaces, the probable cost of maintenance of the protecting cover being duly considered? As a rule in homogeneous earth, such as clay, underground excavation is the most free from water, and therefore from slips; but the enormous force clay possesses upon swelling—and this disturbance of the soil may occur many times by the action of air and moisture—and its contraction upon drying, cause tunnels in clay to be risky undertakings. The chief precautions against failure are to allow plenty of space for the expansion of the clay, reduce the uncovered lengths to the least dimensions, have dry, yet hard and compressible, filling between the arch and the walls and the clay, use the strongest bricks, or material with great compressive strength for the lining, adopt a form giving as equal support as possible in every direction, _i.e._, one closely approaching the circular, leave plenty of weep-holes to prevent any accumulation of water, and thoroughly drain and gently conduct to the proper drainage channels all water as it exudes. CHAPTER III. THE GENERAL EFFECT OF A SLIP IN A CUTTING OR AN EMBANKMENT.—ENUMERATION AND CONSIDERATION OF SOME PROTECTIVE AND REMEDIAL WORKS.—TREATMENT OF THE SLIPPED EARTH.