Man is an air-breathing animal. So soon as his supply of air is cut off he dies. In proportion as that supply is lessened or vitiated, his physical and mental energies fail. One of the first requisites, therefore, in all mining operations is that the ventilation shall be good. To accomplish this end an air current must be established. It is true that into any accessible cavity atmospheric air will rush, but if it be allowed to remain in that cavity without any replacement it becomes dead and unfit to breathe. If, in addition to this, it takes up deleterious gases, like those which escape from the coal measures, it becomes poisoned and dangerous to human life. Hence the necessity of a continuous current. Provisions for such a current are made with the opening into every mine. The separate air compartment of a shaft has already been noticed. In drifts, tunnels, and slopes a part of the opening is partitioned off for an airway, or, what is more common, a separate passage is driven parallel with, and alongside of, the main one. In drifts and tunnels, since the mines there are not deep, air shafts are often driven at some other point above the workings, or slopes are sunk from the outcrop to accommodate the return air from the mine. It is due to the necessity of maintaining an air current that all passages and chambers are driven in pairs or sets in the manner already explained. It has also been explained how the fresh air going in at the carriage ways of the shaft, or other openings, passes along the gangway to its extremity, back along the airway, up to and across the faces of each set of chambers, and then down into the airway again, to be carried to the foot of the shaft and up by the air passage to the surface. But in the larger mines there are many passages besides the main gangway that must be supplied with air, and the current must therefore be divided or split to accommodate them; so these separate currents, taken in this way from the main current, and themselves often divided and subdivided, are called “splits.” The air channels thus branching, uniting, crossing, and recrossing form a most complicated system of ventilation. But the current goes nowhere by chance. Every course is marked out for it. On the fact that it follows that given path depends the lives of the workmen and the successful operation of the mine. Sometimes it becomes necessary to carry two currents of air through the same passage in opposite directions. In that case the passage will either be partitioned along its length, or a wooden box laid through it to conduct one of the air currents. If one air course crosses another, as is often the case, a channel will be cut in the roof of one of the passages, and the lower side of the channel will be closed tightly by masonry, to prevent any possible intermingling of the currents, a circumstance which might prove disastrous. Entrances and cross-headings cut through between parallel passages for purposes of