The hull contained four small double-cylinder engines, one of which actuated the stern-propeller, and the other three the fan-wheels and side-propellers. There were, of course, no furnaces, boilers, or condensers. Two slender pipes ran into each cylinder from suitably placed gas reservoirs, or power-cylinders, as the engineer called them, and that was all. Arnold deftly and rapidly put the parts together, continuing his running description as he did so, and in a few minutes the beautiful miracle of ingenuity stood complete before the wondering eyes of the Circle, and a murmur of admiration ran from lip to lip, bringing a flush of pleasure to the cheek of its creator. "There," said he, as he put the finishing touches to the apparatus, "you see that she is a combination of two principles--those of the Aëronef and the Aëroplane. The first reached its highest development in Jules Verne's imaginary "Clipper of the Clouds," and the second in Hiram Maxim's Aëroplane. Of course, Jules Verne's Aëronef was merely an idea, and one that could never be realised while Robur's mysterious source of electrical energy remained unknown--as it still does. "Maxim's Aëroplane is, as you all know, also an unrealised ideal so far as any practical use is concerned. He has succeeded in making it fly, but only under the most favourable conditions, and practically without cargo. Its two fatal defects have been shown by experience to be the comparatively overwhelming weight of the engine and the fuel that he has to carry to develop sufficient power to rise from the ground and progress against the wind, and the inability of the machine to ascend perpendicularly to any required height. "Without the power to do this no air-ship can be of any use save under very limited conditions. You cannot carry a railway about with you, or a station to get a start from every time you want to rise, and you cannot always choose a nice level plain in which to come down. Even if you could the Aëroplane would not rise again without its rails and carriage. For purposes of warfare, then, it may be dismissed as totally useless. "In this machine, as you see, I have combined the two principles. These helices on the masts will lift the dead weight of the ship perpendicularly without the slightest help from the side-planes, which are used to regulate the vessel's flight when afloat. I will set the engines that work them in motion independently of the others which move the propellers, and then you will see what I mean." As he spoke, he set one part of the mechanism working. Those watching saw the three helices begin to spin round, the centre one revolving in an opposite direction to the other two, with a soft whirring sound that gradually rose to a high-pitched note.