In order to create this circulation of air and make it continuous, artificial means are ordinarily used. The earliest method of creating an artificial air current which should be constant, and one still in use to a limited extent, is that by the open furnace. This is an ordinary fireplace with grate bars, built near the foot of an opening into the mine, and having a bricked-in smoke-flue which leads into the air passage of that opening at some little distance above the floor of the mine. The volume of heat thus passing into the airway will rarefy the air therein, and so create and maintain a strong, invariable, upward current. Sometimes the furnace is placed at the foot of an air shaft a long distance from the main opening, thus making it an upcast shaft. The reverse, however, is usually the case. All air that enters the mine by any opening is usually drawn out at the main shaft or other main entrance. But as the air returning from the working places of the mine is often laden with inflammable gases, it is not allowed to come into contact with the fire of the furnace, but is carried into the shaft by a channel cut into the rock above the roof of the mine. Furnace ventilation in mines in which explosive gases are generated is dangerous at the best, and is now prohibited by the act of 1885. The modern and most common method of creating and maintaining a circulation of air in a mine is by a fan built at the mouth of the air compartment of the shaft or slope. The fan exhausts the air from the mine by the airway, and fresh air rushes in by the carriage way, or any other opening to the surface to restore the equilibrium. Sometimes the fan is used as a blower and forces air into the mine instead of exhausting it. The advantage of this method is that it gives better air to the workmen at the faces of chambers and headings, but the objection to it is that it brings all the smoke and gases out by the main gangway. This is a serious objection, not only making this principal passage unfit to see or breathe in, but making it dangerous also by the presence of inflammable gases. The fan is therefore commonly used as an exhauster. There are various kinds of fans in use at the mines, but the kind generally employed is patterned after Guibal’s invention. It is simply a great wheel without a rim, and instead of spokes it has blades like those of a windmill. It is run by a steam-engine, makes forty revolutions per minute at an average rate of speed, and sends from one hundred thousand to two hundred thousand cubic feet of fresh air per minute into the mine.