And, not only were they in this state of darkness respecting their Maker in consequence of the absence of any form of a direct revelation, but, being absolutely cut off from all knowledge of the remainder of the universe, by the physical structure of their atmosphere, they were also debarred from reaching Him through the medium of His works. The cloud-screen which shelters them from the fierce solar rays is impenetrable to vision, and thus, so far as any knowledge of the sun, and planets, and stars is concerned, they might as well have been a race of blind men. How it was that the canopy over their heads passed regularly in the course of about twenty-three hours and a-half through the two phases of brightness and darkness, was to them an inexplicable phenomenon. All sorts of conjectures, hypotheses, theories, were hazarded, but none were accepted. The phenomenon was not even universal. At one place, near the centre of the continent, and for a considerable distance around it, the alternation of light and darkness followed quite a different law. For, instead of the change taking place at intervals of a few hours, light shone steadily for more than a hundred and twenty days, and was followed by nearly as long a period of darkness. It was an inscrutable puzzle. Some said that on one or two occasions a round and shining body had been dimly seen for a few moments through the mist, and that this might possibly have something to do with the illumination. But the fact was discredited, and the alleged appearance ascribed either to an optical illusion or deliberate mendacity. The observers, accordingly, being invariably treated with either contempt or personal violence, the theory disappeared. Meanwhile great progress continued to be made in all departments of physical science. The various branches of mathematics were extensively and successfully studied, and the Hesperians became most expert geometers. The art of ship-building was soon carried to a high pitch of excellence, and various methods of propelling the vessels through the water were devised by the mechanical engineers. Some such artificial propulsion was almost indispensable, as the prevailing calms rendered the use of sails unavailable. One of the earliest motive powers extensively employed was the expansive force of the vapour of water, raised at a high temperature; and for many hundred years these curious ships were in actual use. I have seen several of them which are still kept in a vast marine museum at Lasondre. The vapour-engines propelled the ships either by means of great wheels furnished with boards which turned in the water, or by the action of one or more screws at the stern, which worked much as the tail of a fish does in shoving the animal along. But the use of the vapour of water as a motor was found to involve a terrible waste of power, and it has been long since abandoned.