And then there was nothing for him to do. Greg handled the navigation skilfully, while Johnny kept radio contact and busied himself in the storeroom, so Tom spent hours at the viewscreen. On the second day he spotted a tiny chunk of rock that was unquestionably an asteroid moving swiftly toward them. It passed at a tangent ten thousand miles ahead of them, and Greg started work at the computer, feeding in the data tapes that would ultimately guide the ship to its goal. * * * * * Pinpointing a given spot in the Asteroid Belt was a gargantuan task, virtually impossible without the aid of the ship's computer to compute orbits, speeds, and distances. Tom spent more and more time at the viewscreen, searching the blackness of space for more asteroid sightings. But except for an occasional tiny bit of debris hurtling by, he saw nothing but the changeless panorama of stars. Johnny Coombs found him there on the third day, and laughed at his sour expression. "Gettin' impatient?" "Just wondering when we'll reach the Belt, is all," Tom said. Johnny chuckled. "Hope you're not holdin' your breath. We've already been _in_ the Belt for the last forty-eight hours." "Then where are all the asteroids?" Tom said. "Oh, they're here. You just won't see many of them. People always think there ought to be dozens of them around, like sheep on a hillside, but it just doesn't work that way." Johnny peered at the screen. "Of course, to an astronomer the Belt is just loaded ... hundreds of thousands of chunks, all sizes from five hundred miles in diameter on down. But actually, those chunks are all tens of thousands of miles apart, and the Belt _looks_ just as empty as the space between Mars and Earth." "Well, I don't see how we're ever going to find one particular rock," Tom said, watching the screen gloomily. "It's not too hard. Every asteroid has its own orbit around the sun, and everyone that's been registered as a claim has the orbit charted. The one we want isn't where it was when your Dad's body was found ... it's been travelling in its orbit ever since. But by figuring in the fourth dimension, we can locate it." Tom blinked. "Fourth dimension?" "Time," Johnny Coombs said. "If we just used the three linear dimensions ... length, width and depth ... we'd end up at the place where the asteroid _was_, but that wouldn't help us much because it's been moving in orbit ever since the Patrol Ship last pinpointed it. So we figure in a fourth dimension ... the time that's passed since it was last spotted ... and we can chart a collision course with it, figure out just where _we'll_ have to be to meet it." It was the first time that the idea of time as a "dimension" had ever made sense to Tom. They talked some more, until Johnny started bringing in fifth and sixth dimensions, and problems of irrational space and hyperspace, and got even himself confused. "Anyway," Tom said, "I'm glad we've got a computer aboard."