To commence with, there is the great wave or tide of force or pressure which surges along the gallery immediately the cloud bursts into flame. How fast does that wave travel? How long is it after the explosion before the shattering effects of it are felt a hundred yards away? To solve that problem electrical contact-breakers are fixed at intervals of fifty yards along the gallery. Each of these consists of a cylinder with a piston inside it something like, shall we say, a cycle pump. The piston, held down normally by a spring, is blown upwards by the force of the explosion. The spring is adjustable, and so it can be arranged that the feeble force of the gun cannot lift the piston, but the more powerful coal-dust explosion which follows can. Thus when the explosion takes place these contact-breakers are operated in succession. The one nearest the seat of the disturbance is operated first; next the one fifty yards farther away; then the one a hundred yards away, and so on. The moments when they work will tell the speed at which the blast travels along the gallery. But it travels with great speed, and so to measure and record the exact moment when each contact-breaker is moved is a matter of no little difficulty. Electricity, however, makes this, like so many other things, comparatively easy. There is an apparatus used in astronomical observatories called a chronograph, which registers, within a small fraction of a second, the moment when a star seems to pass across a wire in the "transit circle," the telescope by which the positions of stars are determined and the exact time kept. The observer sits with his eye to the telescope, watching the apparent movement of the star. In his hand he holds a small "push," pressure on which by his fingers operates a minute pricker, which acts upon a moving strip of paper. The paper travels along with the utmost steadiness and regularity, while a clock drives a sharply pointed pricker on to it every two seconds. Thus the clock marks out the paper into lengths, each of which represents two seconds. But the other pricker, worked electrically by the observer's hand, also makes its mark upon the paper, and so, while the regular marks indicate intervals of two seconds, each irregular one marks the time of a transit or passing of a star across the wire. An examination of the strip subsequently enables the times of a transit to be seen with great accuracy, from the position of the corresponding mark between two of the _regular_ marks.