Rain will percolate through chalk or any open soil until it meets an impervious stratum, or to that place which is in a state of saturation, when the water must either flow away or the level of saturation must gradually rise. This causes rivulets to burst out in places after heavy rain, when the water has had time to percolate, and the rainfall has exceeded the average, but such springs will cease when the local excess has terminated. However, in cuttings it is the flow from fissures that is to be feared, and their size may indicate the quantity of water that may be expected to issue from them. It seems to be generally agreed that the supply of water from chalk is derived from rain, which percolates through innumerable fissures, and that in all rocks, whether limestone, sandstone, granite, sand, or clay, it is by means of the fissures, seams, and veins that the supply of water is obtained from rain, and springs created. With regard to the percolation of water through sand, it may always be expected to be very considerable, and the soil may under certain conditions become water-charged. Mr. Greaves, M. Inst. C.E., has shown by experiments that the average percolation through ordinary top soil is only about one-third of that of sand, but the evaporation from a surface of ordinary soil was about four times more than from a similar surface of sand, and also the amount of percolation in ordinary top earth was small on the whole, and, perhaps, the percolation through ordinary ground would be about 25 per cent. of the rainfall, but 80 per cent. in average sand. Experiments have also shown that the absorbent capacity of sand decreases regularly according to the fineness of the grain, and that “some sandy soils will not absorb more than 20 per cent., but sandy soil containing peat, as moorland, as much as 80 per cent., both computed by weight.” The quantity of water absorbed by loamy soil will vary considerably according as clay or sand preponderates in the mass. Earth may become so mixed with coarse or fine sand that, when saturated, it approaches the condition of a quicksand. The general effect of percolation has been briefly described as follows. Upon water entering the pores of an earth it displaces the air or liquid previously present, forcing the former upwards into the atmosphere, and the latter downwards. Having briefly referred to the percolation of water in cuttings and embankments, the drainage is now considered. It must be either precautionary, _i.e._, to prevent a slip, or remedial, _i.e._, to drain a slip.