In the backing wall was fitted a large mortar of cast steel, which in carrying out the tests served to replace the borehole used in actual mining operations. A pipe for conveying the gas and another for steam were laid on the floor of the chamber, the latter for heating purposes, in order to ascertain whether, in certain cases, an increase in temperature exerts any sensible influence on the inflammability of the explosive mixture. The temperature of the chamber is read off from a thermometer placed at the top of the boiler, its position being indicated by T in Fig. 2. In view of the possibility of the boiler, notwithstanding its strength, bursting, in the event of a violent explosion of the gas, it became necessary to make special arrangements for allowing the operators to observe everything occurring in the testing chamber without being themselves exposed to the consequences of any accident that might ensue. A special shelter was, therefore, erected for occupation by the operators at the moment of the explosion. This shelter, at about a dozen yards away from the boiler, consisted of a chamber protected on the side next the gallery by a stout bank of earth, in which a longitudinal aperture was provided (by means of a lining of boards) at about the height of the face, through which the operators could observe the progress of the tests, without danger. It may be stated, however, that hitherto no accident has occurred, the boiler effectually resisting the force of the explosions. The chamber of shelter likewise contained the gasometer for regulating the supply of gas to the testing apparatus, and the electrical machine for firing the cartridges under test. There being no continuous current of firedamp at disposal, use was made of illuminating gas in preparing the explosive mixtures for the tests. The borehole is charged with the explosive to be fired, and the temperature is regulated by means of the steam pipe. The entrance of the chamber and the two safety apertures in the roof having been closed by disks of paper fastened by paste, the gas is turned on until the desired percentage, has been introduced; the mixture of the air and gas takes merely a short time to effect by diffusion, the difference in density causing the gas to rise on issuing from the jet, which is on the floor of the chamber. The detonating cap is then ignited by the passage of the electric current and the shot fired. The operator, placed in his shelter, can observe, by means of the small lateral windows, whether any flame is produced, and indeed, a little experience will enable him to determine by the sound alone, whether an explosion has ignited the mixture or not.