=The Water contained in the Soil= is divided into moisture and ground or subsoil-water. When air is present in the soil as well as water, the soil is merely moist. Pettenkofer defines the ground-water as that condition in which all the interstices are filled with water, so that, except in so far as its particles are separated by solid portions of soil, there is a continuous sheet of water. The =Moisture= in the soil varies in amount. Open gravel will absorb from 9 to 13 per cent. by weight of water; gravelly surface soil 48 per cent.; light sandy soils from 23 to 36 per cent.; loamy soil 43 per cent.; stiff land and clay soils from 43·3 to 57·6 per cent.; sandy and peaty soils from 61·5 to 80 per cent.; peat 103 per cent. (B. Latham). The moisture being derived from the rainfall on one side, and the ground-water on the other, will vary with the amount of these. Some soils are practically _impermeable_ to water, such as trap or metamorphic rocks, unweathered granite, hard limestone, and dense clay; while others, such as chalk, sand, sandstone, vegetable soils are _permeable_. Commonly the metamorphic rocks and hard limestones present fissures, which render them pervious. The rainfall which does not penetrate the soil flows into the streams and rivers at once, or is re-evaporated. The amount of _percolation_ of rainfall is estimated by an artificial soil-gauge. Most percolation and least evaporation of rainfall occurs from October to March inclusive. The difference between the percolation and rainfall is the loss caused by evaporation and vegetation. The =Ground-water= forms a subterranean sheet of water, which is in constant motion. There is first of all, an irregular rise and fall of the water, according as it receives new additions from the rainfall, or loses a certain amount of its substance by percolation and evaporation; and there is, secondly, a constant movement towards the nearest water-course or the sea. Many towns derive their drinking-water from the ground-water, especially that in the chalk. Thus in Brighton there are no streams; but wells are dug in the South Downs about 150 to 180 feet deep down to the level of the subterranean water. Then long adits are tunnelled, parallel to the coast at or near the level of this water, which is thus intercepted on its way towards the sea, and pumped up to supply the town. In Munich, Pettenkofer reckoned the rate of movement of the ground water towards the outlet as 15 feet daily. It is impeded by impermeability, or a deficient slope of the soil. The roots of trees also greatly impede its flow. _The level of the ground-water_ is constantly changing (see Fig. 7). The alteration in level may be only a few inches either way, while in some parts of India it is as much as 16 feet. The level is generally lowest in October and November, highest in February and March.