NATURAL VENTILATION.
This is the first and simplest mode, and one which is naturally suggested by the ordi nary currents of the air. It is the method always made use of in all new mining dis tricts, except where the seams lie deep beneath the surface. As before stated, the air is carried into the mine at the lowest level, and returned to the atmosphere at a higher level, where the air is more rarefied, and where the atmospheric pressure is the least. Under these circumstances, when the avenues of the mine are regular and proportional, an extensive circulation is maintained, liable to derangements only from sudden changes of the weather and variation of the temperature of the atmosphere.
In deep mines, or most mines carried below water-level, this mode is not available to any great extent. It is very difficult to make any arrangement, under such circum stances, to keep up a current. The levels being equal in many cases, and generally
nearly so, the air is in equilibrium, the atmospheric pressure and rarefaction being equal, and there being no tendency to motion. It is true the temperature in most deep mines is higher than the ordinary temperature of the atmosphere, and that the pressure is greater at 500 feet of depth than at the surface; but their effect is equal, and exerted on all portions of the mine alike: consequently, some force must be employed to create a movement and propel a current of air through the mine in a given direction. This is generally done by heat, or the rarefaction of the air in the upcast shaft, which creates a rapid upward movement of the mine-vapor, and, consequently, a downward movement of the atmosphere to fill its place. This method is known as furnace-ventilation.