The general idea of these stations is as follows. The building is centrally situated in the district which it is intended to serve, and in it are kept an ample supply of the necessary appliances, in the shape of breathing apparatus, which enables men to walk unhurt through poisonous gas, reviving apparatus, by which partially suffocated men can be brought round again by the administration of oxygen, together with quantities of that valuable gas in suitable portable cylinders. Everything which forethought can suggest as even possibly useful in an emergency is kept in a constant state of readiness. And all the while a swift motor car stands ready to carry them to the scene of operations. But the appliances are of little use without men to work them, who know them and can trust them. The case of David, who felt able to do better work with his sling and stone than in all the panoply of Saul's armour, because he "had not proved it," is typical of a universal human instinct. A man feels safer unarmed, or simply armed, than he does with the most elaborate weapons in which he has not learned to have confidence. And therefore the men who may be called upon to work this apparatus are first taught to have confidence in it. Each station has its instructor, who is usually also the general superintendent of the station, and "galleries" in which the instruction can be carried out. Volunteers are called for in each colliery and a number of the most suitable men are chosen to undergo training, preference being given, very naturally, to those who are already trained, as fortunately so many workmen are nowadays, in ambulance work. These chosen men then repair at intervals to the station to undergo a proper course of instruction. The instructor, often an ex-non-commissioned officer in the Royal Engineers, accustomed, therefore, to engineering matters, and also to systematic discipline, there puts them through a course of drill the object of which is to teach them to work together as a squad under the orders of a properly constituted chief. Thus when called upon in some emergency there will be no confusion, but each man will know what to do, and a few short words of command from the chief will serve better than the long explanations which would be necessary with an undisciplined body. It welds the individual men, as it were, into a smoothly working machine, thereby increasing the efficiency of the whole. And arrangements are made whereby, should the leader fail, another man steps into his place of authority at once and without question. Then, having thus brought them under a suitable discipline, the instructor takes his men into the experimental gallery. This may be described as a long, low, narrow shed, in which are timber props and beams, rails on the floor, heaps of coal, all things, in fact, which may tend to make it closely resemble the actual workings of a coal-mine after they have been shaken and shattered by the force of an explosion.