Spring being the driest season in England and autumn the wettest, October and November being the months of heaviest rainfall, slips are more probable in the latter than the former season; but the first heavy and continuous rainfall after a period of estival drought is that particularly to be feared, or the first rainy weather after a dry period irrespective of the season of the year; but serious slips may not occur for many days or until the expiration of even a month or two after such rainfall, as the ground waters require time before they percolate to or reach the site of a cutting, and, therefore, all danger may be thought to be past when it is steadily approaching. The autumnal rains have to replace the moisture that has evaporated during the summer, and this may not, and usually does not if it slowly proceeds, produce instability in earthwork; but immediately the rainfall approaches or becomes in excess of the power of the natural absorption of the soil, the rain must flow away, for the earth being fully charged cannot contain it, the surface becomes wet or the mass soaked, according to the degree of the permeability of the soil, and the quantity of rain necessary to produce saturation; therefore, the state of the earth that induces a slip is that most desirable to know; this cannot be absolutely established in every case, for it depends upon so many influences, and obviously varies according to the character of the earth, the varieties and conditions of which are practically infinite. In countries that have dry and wet seasons, which cause the earth to become parched and then to be rapidly saturated, mere surface waters to become streams, and rivers torrents, slips are probable soon after the commencement of the rainy season. When frost follows rain or a fall of snow, and the latter has descended upon a frozen surface and a thaw sets in, particularly if it be accompanied by a warm wind causing it to be very rapid in action, the earth is severely tried, for the frozen water in the ground becomes suddenly liberated, while the surface is in a state of saturation. Probably the worst event that can occur for causing floods is when a sudden and rapid thaw follows a heavy snowfall upon frozen earth, as then the snow will melt, and water cannot gently percolate the earth, as the surface will be in a more or less frozen and impermeable condition, and the snow-water consequently must flow away. Should any excessive or violent rainfall succeed a period during which the heat of the sun has caused fissures in the surface of the earth, allowing water to enter, the state of the soil is favourable to movement. Land has also become unstable in mountainous countries because a district has been deforested, or tree-protection much reduced, movement of the earth usually happening after the first heavy rains or thaw succeeding frost or snow.