"I think so," Scotty said. "The denser the stuff the wave strikes, the faster it moves. Like sound waves. I mean, sound moves faster in water than in air, and faster in a steel rail than in water. Is it the same?" "Just about," Zircon agreed. "The shock waves radiate away from us, through the earth, and eventually reach the recorders on the other side of the mountain. You can see what happens, I think. Waves will arrive at different times, depending on the path they took and the kind of material they went through." Rick nodded. "So if there's molten rock, or magma somewhere in the way, the shock wave that goes through it will slow down and arrive at the recorder later?" "That's it. The tracings we get can be analyzed to give us a kind of cross-sectional look at the mountain. You see, we know how fast the waves travel through different kinds of earth structure. Also, we will know the point of the explosion and the location of the recorder for each shot. Which reminds me. We'd better get out the equipment and locate ourselves precisely." "How?" Rick asked. "What will we use for landmarks?" "The top of the mountain, for one, and if you'll look carefully to a point slightly south of east between those two banana palms, you'll see the top of the control tower at the airport." Rick shook his head. "Good thing you're with us. I completely forgot to watch for landmarks." "That was the first thing I had in mind in looking for a spot," Zircon told him. The transit gave a precise angle between the two landmarks. Zircon drew a line on the map connecting the southern tip of the mountain and the airport tower. Then, with that as his base line, it was easy to draw two lines at the correct angles from each of the points. The transit's position was where the two lines intersected. By the time the scientist had finished, it was nearly one o'clock. The three walked to the detonator. "Pull the handle up," Zircon directed. Rick did so. "I'll count down from ten seconds. Push down on zero." It was like the countdown for a rocket firing, Rick thought. Zircon called out the time starting at one minute, then called off the last ten seconds. As he reached zero, Rick pushed the handle home. The dynamite went off with a roar that sent leaves and dirt flying, and Rick felt the shock wave slam against his ears with stunning force. "Open your mouth next time," Zircon said. "I forgot to warn you." He was already reeling in the wire. "Let's get going. One mile farther on for the next shot." At the next station the same procedure was repeated, but before it was time, there was a far-off explosion. Zircon looked at his watch. "Brad Connel. Right on time." In another fifteen minutes there was an even more distant sound as David Riddle's first shot went off. They ate their lunch and listened to the echo off the mountain.