“But it’s simple enough — ”
“Not for me.”
It’s simple for anyone. You know how the layer of hot air just above a road on a sunny day bends sky light back upward at a slight angle, since the hot air is less dense and the light travels faster in it; you see the sky reflection and tend to interpret it as water. You get more extensive mirages sometimes even on Earth, but they’re all based on the same thing — a ‘lens’ or ‘prism’ of colder or hotter air refracts the light. It’s the same here, except the gravity is responsible; even hydrogen decreases rapidly in density as you go up from Mesklin’s surface. The low temperature helps, of course.”
“All right if you say so; I’m not a — ” Lackland got no chance to finish his remark; Rosten cut in abruptly a and grimly.
“Just how fast does this density drop off with altitude?”
The meteorologist drew a slide rule from his pocket and manipulated it silently for a moment.
“Very roughly, assuming a mean temperature of minus one-sixty, it would drop to about one per cent of its surface density at around fifteen or sixteen hundred feet.” A general stunned silence followed his words.
“And — how far would it have dropped at — say — three hundred feet?” Rosten finally managed to get the question out.
The answer came after a moment of silent lip movement. “Again very roughly, seventy or eighty per cent — probably rather more.”
Rosten drummed his fingers on the table for a minute or two, his eyes following their motions; then he looked around at the other faces. All were looking back at him silently. “I suppose no one can suggest a bright way out of this one; or does someone really hope that Barlennan’s people can live and work under an air pressure that compares to their normal one about as that at forty or fifty thousand feet does to ours?”