The flitter hung poised in air, motionless, upon softly hissing under-jets. Cloud knew to a fraction his height above the ground. He knew to a fraction his distance from the vortex. He knew with equal certainty the density of the atmosphere and the exact velocity and direction of the wind. Hence, since he could also read closely enough the momentary variations in the cyclonic storms within the crater, he could compute very easily the course and velocity necessary to land the bomb in the exact center of the vortex at any given instant of time. The hard part--the thing that no one had as yet succeeded in doing--was to predict, for a time far enough ahead to be of any use, a usably close approximation to the vortex's quantitative activity. For, as has been said, he had to over-blast, rather than under-, if he could not hit it "on the nose:" to under-blast would scatter it all over the state. Therefore Cloud concentrated upon the dials and gauges before him; concentrated with every fiber of his being and every cell of his brain. Suddenly, almost imperceptibly, the Sigma curve gave signs of flattening out. In that instant Cloud's mind pounced. Simultaneous equations: nine of them, involving nine unknowns. An integration in four dimensions. No matter--Cloud did not solve them laboriously, one factor at a time. Without knowing how he had arrived at it, he knew the answer; just as the Posenian or the Rigellian is able to perceive every separate component particle of an opaque, three-dimensional solid, but without being able to explain to anyone how his sense of perception works. It just _is_, that's all. Anyway, by virtue of whatever sense or ability it is which makes a mathematical prodigy what he is, Cloud knew that in exactly eight and three-tenths seconds from that observed instant the activity of the vortex would be slightly--but not too far--under the coefficient of his heaviest bomb. Another flick of his mental trigger and he knew the exact velocity he would require. His hand swept over the studs, his right foot tramped down, hard, upon the firing lever; and, even as the quivering flitter shot forward under eight Tellurian gravities of acceleration, he knew to the thousandth of a second how long he would have to hold that acceleration to attain that velocity. While not really long--in seconds--it was much too long for comfort. It took him much closer to the vortex than he wanted to be; in fact, it took him right out over the crater itself.