38 MINE VENTILATION. $14. extinction by asphyxia, caused by the admission of a large quantity of gas. It was also noticed that explosion takes place after from five to ten seconds when the velocity is 9.84', and after from two to five seconds when the speed is 19.68'. “With the Mueseler lamp, out of a one hundred and fifteen experiments, there were twenty-one cases of complete extinction at a velocity of 19.68' per second. “The Morison lamp was considered of very complicated construction, and as giving a very bad light in stagnant air. Out of eleven experiments at a velocity of 19.68′ per second, these lamps caused neither exterior explosions, nor any inflammation of gas in the exterior cylinder. 66 Rapid currents of air are dangerous when their action manifests itself by the crushing of the flame upon the wicks: indeed, the relative security of the Mueseler lamp does not depend alone on the smallness of the section of the chimney at the top, or on its height, but rests essentially in the regularity of the draught." The North of England Institute of Mining Engi neers, which has been so instrumental in the furtherance of mining knowledge, appointed a committee, who rendered in their report the following concerning the velocity at which the various lamps would explode: This rather conflicts with the Belgium Report, as they claim the Mueseler lamp passes flame as easily as the Davy. 14 a. Fire-damp may be detected by the aid of a Davy or other safety-lamp. The following, taken from the Galloway Royal Society's Journal of 1876, gives the various appearances of the lamp-flame when brought in contact with air mixed with fire-damp: "The wick of the lamp, having been carefully trimmed, was drawn down until the flame presented the appearance of a small blue hemisphere about oneeighth of an inch high, one-quarter inch diameter at the base, and having a conical speck of yellow in the middle near the top. "A mixture of 1 part of marsh-gas with 16 parts of air gave a voluminous waving, spindle-shaped blue cap 3" high. "1 part of marsh-gas with 18 parts of air gave a cap 2" high, which burned more steadily. "1 part of marsh-gas with 20 parts of air gave a cap 40 16 MINE VENTILATION. $15. 15" high, with nearly parallel sides to about two-thirds of its height, and then tapered to a point at the top. 66 "1 part of marsh-gas with 25 parts of air gave a conical cap from to " high. "1 part of marsh-gas with 30 parts of air gave a conical cap " high. 66 "1 part of marsh-gas with 40 parts of air gave a conical cap " high. "1 part of marsh-gas with 50 parts of air gave a faint cap "high, the top having the appearance of having been broken off. "With 1 part of marsh-gas and 60 parts of air, it was hardly possible to distinguish any thing above the small oil-flame." CHAPTER IV. PHYSICAL PROPERTIES OF AIR IN MOTION. 15. WIND is air in motion; and, as air is matter, it is subject to the laws which govern matter. No particle of matter possesses within itself the power of changing its existing state of motion or rest. When a body is at rest, a force is required to put it in motion; and, when once put in motion, it would continue to move on for ever if a force of some kind were not opposed to it to arrest its movement. This passive property of air is called its inertia, and may be defined as opposition to change, either from motion to rest, or vice versa. If the air, then, had no inertia, it would not require force to give it motion, nor could it require momentum. The sailing of ships, the windmill, the tornado, are familiar examples of the power of moving air, and, consequently, proofs of its inertia. In order to pass air through a mine, certain force must be expended; and this force is what we are now about to examine. It involves a consideration of the resistances to be overcome, such as area of the airways, obstructions in the airways, and the friction against the walls or sides of the airways. When air travels the galleries of a mine, it rubs against all the exposed surfaces. This rubbing gives rise to the resistance called "friction." Friction of air in mines is so great, that, out of every ten parts of power employed for the ventilation of a mine, about nine of the ten are used in overcoming the resistance to ventilation. As the air journeys through the mines from the downcast to the upcast, it not only meets the rubbing-surface, or sides, but often encounters short turns, brattices, etc., which adds greatly to the total resistance to be overcome. In turning sharp corners, the air strikes square 42 MINE VENTILATION. $16. against the face, and rebounds; thus hindering the progress of the air following, which, in turn, goes through the same operation, and hinders the air following it. It is therefore preferable to have well-rounded bends of large radius, as they produce little resistance in comparison with elbows or square bends. The con sideration of the movements of air in mines or confined passages involves its density, the area, length, and perimeter of the airways, also the velocity with which the air travels. 16. To find the perimeter of an airway, we must add together the bounding-lines. The perimeter of a square airway 6' × 6′ is therefore 6+ 6 + 6 + 6 = 24′. The perimeter of a circle is its circumference, and is 3.1416 times its diameter: hence the perimeter of an airway six feet in diameter is 18.8496'. The sectional area of an airway is found by multiplying its height by its width. Thus the area of an airway 5' x 6' is 30 square feet. The rubbing-surface is found by multiplying the perimeeter by the length of the airway. Problem. What is the rubbing-surface of an airway 500' long, with an area of 6' x 6'? Solution.6+ 6 + 6 + 6 = 24'; then 24' X 500 = 12,000 square feet. Ans. As friction increases according to the rubbing-surface, |