There are several makes of these, but a description of the particular apparatus used at the two stations mentioned will serve as an illustration. The purpose, of course, is to give the wearer an atmosphere of his own, which he can carry about with him, and which will render him quite independent of the ordinary atmosphere and quite indifferent to the poisonous nature of the gases around him. To this end his mouth and nostrils must be cut off from the outer world altogether. There are two ways of doing this. In the one there is used a helmet, or perhaps mask would be the better term. This fits right over the man's face, an air-tight joint being made between the helmet and his head by means of a rubber washer which can be inflated with air. The inflation is accomplished by squeezing a rubber ball on the right-hand side of the helmet. In the centre is a glass window through which he can see easily, and since this is apt to become clouded by the dampness of his breath there is a wiper inside, which can be turned by a knob on the outside, so that by simply turning his knob with his hand he can clean the window at any time that may be necessary. Two soft pads inside the helmet bear one on the man's forehead and the other on his chin, and these, working in conjunction with a strap which passes right round the back of his head, keep the thing firmly in position. In addition there is combined with the helmet a leather skull-cap which, being continued down behind, gives good protection to the head and neck. The other form of apparatus consists of a mouth-piece and nose-clip. The mouth-piece, as its name implies, fits in the man's mouth, being supported and kept in position by a strap passing behind the back of his head. Combined with it is a little screw clip which closes his nostrils. The man also wears a leather skull-cap, from which straps depend to bear the weight of the mouth-piece and its attached tubes, so that the weight does not fall upon his mouth. Either of these arrangements, it is clear, cuts him off from communication with the outer air, but that is only half the problem, for he must be given a substitute or he will be suffocated. This part of the appliance he carries, knapsack fashion, upon his back. First there is a rectangular case, called the regenerator, with, below it, two small cylinders of compressed oxygen. A suitable arrangement of pipes connects these together, and to the helmet or mouth-piece as the case may be.