WATERFALLS AND STEAM-JETS.
These modes are now considered obsolete, and are not used, except on rare occasions, when steam can be produced abundantly and cheaply, or when water can be used with out repumping, and drained off by some adit level. Waterfalls were much used when natural ventilation was the only other means employed to start a column of air. If a large body of water is suddenly let down a pit, it compels the air to move before it and follow after it; and thus, when the air has become stagnant or in equilibrium by some change in the weather and temperature, the column is started in its proper direction, and may, in shallow shafts, be kept moving by the ordinary means of natural ventila tion. It is sometimes used also in starting the furnace-fires, or where weak ventilation is used, because it is much more difficult to put in motion a long column of air than to keep it moving afterwards.
The steam-jet is almost valueless in deep shafts if used alone, though it answers very well in shallow ones, and is an aid in deep ones in connection with the furnace. The steam-jet acts like a steel spring. Its action is in its immediate vicinity, and its energy is soon exhausted. On being released from tension, it suddenly springs upward, and of course moves the air quickly in its vicinity, but its expansion is momentary, or its force confined to a limited area, and does not exert its propelling power to any distance up the shaft. Therefore, if the shaft is deep the steam cools and condenses before reaching the surface, and not only loses its motive force, but actually falls back and retards the column. In a shallow shaft this would not happen, as the steam would reach the surface before condensation took place ; and even in deep shafts, where fur naces are used in connection with steam-jets, the action of the latter may be beneficial. The energetic action of the steam on being released from pressure in the steam-boiler moves the air rapidly, while the heat of the furnaces, ascending the shaft, prevents the steam from suddenly condensing. But neither the one nor the other is capable of forcing the air, if much pressure is required, through the contracted avenues of the mine. This can be done only by mechanical means ; but in all cases, whatever motive power be used, the more contracted the air-passages may be, the more power is required to produce a given amount of ventilation, or to pass a given number of cubic feet of air per minute through the mine. If it requires a force equal to 50 horse-power to
move a column of 100,000 cubic feet of air per minute through the avenues of a mine, it would require 8 times that amount of force, or 400 horse-power, to increase the ven tilation to 200,000 cubic feet per minute through the same mine.
If the main avenue of the downcast shaft and the inlet air-courses be equal to 100 feet in area, or 10 feet in diameter, the air would require to travel 1000 feet per minute in order to pass 100,000 cubic feet per minute with the power proposed. But if the area of the air-courses be contracted in any part of the mine to one-half the size pro posed, or about 7 feet diameter, the power required to propel the column is increased in proportion to the contraction, but not in the ratio of 8, as before stated, since the con traction of the air-courses impedes the progress of the air in proportion to their length as well as in proportion to their area.
The formula, as found by experiment and practice, is this :—" The pressure per unit of surface, or head of air-column, required for the propulsion of air through a contracted passage, is proportional to the square of the velocity, or to the square of the quantity of air passing in a given time, divided by the square of the area of the air-course.
"The pressure required to propel air through a contracted passage is proportional to the length of the passage, or to the perimeter of the section of the passage, under equal circumstances." The friction of air through a rough or irregular passage is much greater than through a smooth and uniform one. Unlike water, which flows through a rugged channel of coarse stone-work with as little friction as through glass pipes, it is retarded by fric tion with every acute angle and every projecting point ; and though these interruptions are insignificant as items, they exert a great influence combined; and the "drag" of a column of air several miles in length is much greater through irregular and rough channels than through comparatively smooth and uniform passages.