The contact-makers are of two kinds. In one the pressure of the explosion raises a piston which completes a circuit allowing current to flow through the very fine wire which prevents the fall of the hammer. This fine wire being fused by the current, the hammer falls and does its work. The other kind, which are used when the force of the explosion is not enough to raise a piston, is operated by one of the tinfoil circuit-breakers. A magnet, being energised by current passing through the foil, holds up a curved bar over two cups of mercury. Broken by the heat of the explosion, the foil cuts off this current, de-energises the magnet, and allows the bar to fall with its ends in the mercury. This completes another circuit, permitting current to pass to the fine wire, whereby the hammer is released. By connecting a bottle to a contact-maker at a distance the sample can be obtained at any desired period of the explosion. If, for instance, the sample is to represent the immediate products of combustion, it is placed near to the contact-maker. Then the sample is drawn in practically at the moment of explosion. If, on the other hand, it is the after-damp that is to be sampled, then the bottle would be connected to a contact-maker a long way from the seat of the explosion, with the result that its glass cap would not be broken until some considerable time had elapsed after the explosion has passed the bottle. The time also during which the bottle is drawing in its sample can be adjusted by varying the length of the cord to which the weight is attached. And last of all must be mentioned the employment of a kinematograph, capable of taking twenty-two photographs per second, for observing the effects at the ends of the gallery (see illustrations). Thus records are obtained of the force and heat of the explosion, its mechanical and thermal effects upon the walls of the gallery, or, if it were in a real pit, the effects which it would have in shaking and in heating the workings, and the men labouring in them. This and the analysis of the gases producing and produced by the explosion, derived from the contents of the bottles, give sound data upon which can be built up reliable theories as to the nature of colliery explosions and the way to prevent them, results which could be obtained in no other way. No one can help being struck with the thoroughness and ingenuity of the means adopted to these ends, and it is no exaggeration to say that it is a splendid example of thoroughly scientific methods applied to an important industrial investigation. It will be interesting to conclude this account with a brief mention of some of the results to which these painstaking efforts have led. First in importance the fact is placed beyond doubt that coal-dust, which in bulk will only burn slowly, will, when well mixed with air, explode. And no combustible gas need be present to aid in the explosion.