“That’s where the wave of electrical energy enters. It goes into the interior of the machine through one of those tiny slits which you see. Once inside, the wave strikes a magnetic coil about a mirror, which swings when the energy acts upon it, and throws a beam of light down that scale.” He pointed to the opposite wall. There, extending from one side to the other of the room, some fifty feet in all, stretched a scale like a foot rule suddenly grown gigantic. Its space was covered with divisions, a big zero in the middle and numbers running up from zero into the hundreds of thousands and millions on either side. Just at the zero point rested a long narrow beam of light. “You see that beam,” Tom went on. “When the waves come into the machine, they go through as I explained, the machine stops, and the light goes up or down the scale. The distance that it goes shows how far away the wave started. The slit through which the wave comes shows the exact direction from which it comes, and we can get that easily because the machine stops as the wave goes through. Then, by means of a certain amount of mathematics, we hope to be able to find just where a wave comes from. We can adjust the machine so that it will register anything from a wireless telegraph message through a radium discharge to the enormously powerful waves which ‘the man’ uses. We have it adjusted now for the waves which ‘the man’ uses in destroying battleships. We know something of them from the way in which they charged the reflectoscopes. That’s the whole thing.” “One thing more,” I said inquiringly. “If ‘the man’ destroys a battleship, does the machine stop and the beam of light run down the scale.” “Yes,” answered Tom. “That’s just what it does.” “All right,” I said. “Now, we’ll start up,” remarked Tom. “Turn off the lights, throw off the inner insulation,” he commanded, turning to Jones, who obediently threw a couple of switches. We were left in partial darkness. On the long scale, on the opposite side of the room, the single line of light rested at the centre, illuminating the zero. There was a shaded incandescent in one corner, which threw no light on the black wall where stood the scale, but gave a dim radiance sufficient to reveal the belt of polished metal as it swiftly revolved about the mass within. Dorothy sat near the apparatus. Jones was puttering with something at one end of the scale, and Tom and I sat side by side, watching the whole scale. Suddenly the beam swept swiftly far up the scale, fluttered for a moment and rested on a point. The moving belt stopped with a slight click.