Another purpose that their huge chests served was to bear the strain of the great muscles that controlled their arms, and of the powerful engines that, strapped on to them, gave the strong and swift beat to their wings. Their arms were moulded on lines of similar strength; for they had to bear the strain of the forward stroke of the wing, whilst also having to manipulate by means of the long and sinewy fingers its great folds in the backward sweep; and, when more expanse was needed during calmer weather or when resting in the sky, the arms had to thrust out and to bear long rods that in their turn bore expansions of the wings like the studding-sails of a ship. The thumb of each hand was kept free for the management of their breast and shoulder-engines; and it had become by exercise more vigorous and more flexible than the ordinary human thumb. In each armpit was carried a small engine that could be used either as subsidiary to the great breast-engine or for the partial or complete furling of the wings. Beside it was a storage-battery, in which could be generated by the movements of the arm more electricity to supply the central power, thus enabling them to extend their flight through long periods. If they became tired they could expand and inflate their wings with a gas made much warmer by the heat of their bodies than the surrounding atmosphere; then throwing themselves on their backs they could rest or rise in the air as on a balloon. In slow or ordinary flight, or when the wind was not high, they could steer themselves rudely by manipulating the outer folds of their wings with their fingers. But if they wished to fly swiftly, or in some other direction than the wind would bear them, they could push out a tail-like membrane of irelium from between the feet and move it hither and thither by the sinewy power of the heels. The great toe of each foot was also much developed by long use for stretching out and managing the wings; it had become more like a thumb, capable of seizing and manipulating cords or membranes. It was this, added to the lightness of their bodies, that gave them their springy gait, and made them seem when they walked as if they scarcely touched the ground; they could skim like a bird close to the earth by using only the outer folds of their wings and the tip of the great toe for propulsion.