And the same principle was applied to the circuit-breakers of this artificial mine. Normally, current flows through the circuit-breaker, but the lifting of the piston breaks the circuit (whence the name of the contrivance), and that breaking of the circuit and consequent cessation of the current operates the chronograph. By a cleverly constructed device, the details of which are too complicated to set out here, each circuit-breaker in turn makes its mark on the same strip, so that the distances apart of these marks show the time taken by the force of the explosion to travel fifty yards. Meanwhile the clock goes on making its regular marks (in this case every half-second), so that they form a scale by which the other intervals can be measured very exactly. The chronograph used here is more accurate than that in use at Greenwich Observatory, the reason being that in this case the recording currents are sent mechanically by the contact-breakers operated by the explosion itself, while in the case of the astronomer the human element comes in. To watch a moving speck of light and to tell exactly when it crosses a fine line is by no means easy, and so to tell the time within a tenth of a second, is about the limit of possible accuracy. The instrument we have been referring to, however, can register the time which a gaseous wave moving 3000 feet per second takes to travel fifty feet. In other words, the circuit-breakers can be operated so fast that when only a sixtieth of a second intervenes between the action of one and that of the next the chronograph can duly record the fact. The records of the chronograph can be made in two ways: one by a pen on a piece of paper tape, and the other by a scratch on a piece of smoked paper. So by that means the progress of the "force" of the explosion can be measured. It is necessary also to time the movement of the "heat" of the explosion, for the two may not travel together, and the difference between them may let in some light as to the nature and behaviour of the explosion. So for this second purpose a second set of circuit-breakers are used. Each of these consists of a strip of thin tinfoil stretched across the gallery. Being placed edgeways to the moving current of gas, the force of the explosion has no effect upon it, but the heat instantly melts it. Normally, current flows through the strip, and so the melting is signalised by the cessation of the current, which event is recorded by the chronograph.