The level of the ground water depends upon the underlying strata, and also upon the movements of the subterranean water bed. The relative position of the impermeable underlying strata varies in its distance from the surface soil. In marshy land the ground water is at the surface; in other places it can be reached only by deep borings. The source of the ground water is the rainfall, part of which drains into the porous soil until it reaches an impermeable stratum, where it collects. The movements of the ground water are in two directions--horizontal and vertical. The horizontal or lateral movement is toward the seas and adjacent water courses, and is determined by hydrostatic laws and topographical relations. The vertical motion of the ground water is to and from the surface, and is due to the amount of rainfall, the pressure of tides, and water courses into which the ground water drains. The vertical variations of the ground water determine the distance of its surface level from the soil surface, and are divided into a persistently low-water level, about fifteen feet from the surface; a persistently high-water level, about five feet from the surface, and a fluctuating level, sometimes high, sometimes low. =Ground Air.=--Except in the hardest granite rocks and in soil completely filled with water the interstices of the soil are filled with a continuation of atmospheric air, the amount depending on the degree of porosity of the soil. The nature of the ground air differs from that of the atmosphere only as it is influenced by its location. The principal constituents of the air--nitrogen, oxygen, and carbonic acid--are also found in the ground air, but in the latter the relative quantities of O and CO2 are different. AVERAGE COMPOSITION OF ATMOSPHERIC AIR IN 100 VOLUMES Nitrogen 79.00 per cent. Oxygen 20.96 " Carbonic acid 0.04 " AVERAGE COMPOSITION OF GROUND AIR Nitrogen 79.00 per cent. Oxygen 10.35 " Carbonic acid 9.74 " Of course, these quantities are not constant, but vary in different soils, and at different depths, times, etc. The greater quantity of CO2 in ground air is due to the process of oxidation and decomposition taking place in the soil. Ground air also contains a large quantity of bacterial and other organic matter found in the soil. Ground air is in constant motion, its movements depending upon a great many factors, some among these being the winds and movements of the atmospheric air, the temperature of the soil, the surface temperature, the pressure from the ground water from below, and surface and rain water from above, etc.