"How?" asked Dorothy, curiously. "Fix me up a detector and follow up one of those beams. Find its frequency and direction, first, you know, then pick it up outside and follow it to where it's going. It'll go through anything, of course, but I can trap off enough of it to follow it, even if it's tight enough to choke itself," said Seaton. "That's one thing I got from that brain record." * * * * * He worked deftly and rapidly, and soon was rewarded by a flaring crimson color in his detector when it was located in one certain position in front of one of the meters. Noting the bearing on the great circles, he then moved the _Skylark_ along that exact line, over the reflectors, and out beyond the island, where he allowed the vessel to settle directly downwards. "Now folks, if I've done this just right, we'll get a red flash directly." As he spoke the detector again burst into crimson light, and he set the bar into the line and applied a little power, keeping the light at its reddest while the other three looked on in fascinated interest. "This beam is on something that's moving, Mart--can't take my eyes off it for a second or I'll lose it entirely. See where we're going, will you?" "We are about to strike the water," replied Crane quietly. "The water!" exclaimed Margaret. "Fair enough--why not?" "Oh, that's right--I forgot that the _Skylark_ is as good a submarine as she is an airship." Crane pointed number six visiplate directly into the line of flight and started into the dark water. "Mow deep are we, Mart?" asked Seaton after a time. "Only about a hundred feet, and we do not seem to be getting any deeper." "That's good. Afraid this beam might be going to a station on the other side of the planet--through the ground. If so, we'd have had to go back and trace another. We can follow it any distance under water, but not through rock. Need a light?" "Not unless we go deeper." For two hours Seaton held the detector upon that tight beam of energy, traveling at a hundred miles an hour, the highest speed he could use and still hold the beam. "I'd like to be up above watching us. I bet we're making the water boil behind us," remarked Dorothy. "Yeah, we're kicking up quite a wake, I guess. It sure takes power to drive the old can through this wetness." "Slow down!" commanded Crane. "I see a submarine ahead. I thought it might be a whale at first, but it is a boat and it is what we are aiming for. You are constantly swinging with it, keeping it exactly in the line." "O.K." Seaton reduced the power and swung the visiplate over in front of him, whereupon the detector lamp went out. "It's a relief to follow something I can see, instead of trying to guess which way that beam's going to wiggle next. Lead on, Macduff--I'm right on your tail!"