Again half of them would fold their wings and be borne by the other half with a speed and lightness almost as great as when flight was unburdened. All mere earthly amusements and exercise had ceased when the secret of flight had been mastered. For generations their biologists, anatomists, and physicists had studied the wing-power of animals with a view to the practical mastery of it for the Limanorans themselves. Their chief guide towards the analysis was the study, not of birds or insects, but of the bat. They measured the force of the strong chest muscles that enabled it to move its wings with such rapidity; this could be done to a nicety by means of their refined instruments for gauging latent power, whether in tissue or nerve or muscle. They calculated the number of beats it could make in a minute. They measured the spread of the wings and the weight of the body. Thus they came to an almost constant equation of wing-power to size and weight. The physicist and mechanic were then called in; but they would have been helpless without the new metal, irelium, and their power of concentrating great power into small space. This metal was extracted by a process from common earth, but could also be found pure some miles down in the earth. It was perhaps the first essential to the rapid advance of their civilisation because of its extreme lightness and strength, and still more its wonderful flexibility and elasticity when mixed with certain proportions of other substances. It could be made into the most delicate membrane, fine as gauze and yet tough and resistant as leather. It formed the material of their most massive engineering works, and of their lightest draperies and garments. Nothing could surpass its adaptability to all purposes of civilisation. It was out of this that they were able to make their wings which seemed so fragile and yet could bear the force of the wildest storms. It would stand stiffly on its framework against the strongest pressure, and yet could be expanded balloon-wise from within. The only means of disabling these wings was perforation by a hard, sharp point. This could never occur in the air except from the beak of a bird; and then they could still use their spread as a parachute to break their descent. Another quality this metal had was its transparency, and their flight was somewhat concealed from the sight of gazers below by the colour they took from their atmospheric surroundings; it was difficult to distinguish them from a floating cloud or a darker patch of grey or blue sky. The wings could be easily folded or expanded, so flexible was the material; and, when the Limanorans landed from their flight, scarcely a minute elapsed before the huge sails, framework and all, had been furled and had disappeared in the ordinary outline of their bodies.