Then, somewhere quite close at hand, a deep-toned gong sounded a single stroke. Instantly the agent looked up; and Smith saw that he was inspecting the interior of a large engine-room. He had time to note the huge bulk of a horizontal cylinder, perhaps fifty feet in diameter, in the immediate background; also a variety of other mechanisms, more like immensely enlarged editions of laboratory apparatus than ordinary engines. Smith looked in vain for the compact form of a dynamo or motor, and listened in vain for the sound of either. Then, in swift succession, came two strokes on the unseen gong, followed by a shrill whistle. Smith's borrowed eyes became fixed upon that group of dials again. Their indicators began to shift, some rapidly, some slowly. Once the agent gave a swift glance through a round window--the place seemed to be lighted by ordinary daylight--and Smith saw something unrecognizable flit by. A little further progress, and then came three strokes on the gong, followed by a low thrumming. In response to these, the agent deliberately picked out two levers, and pulled them down. When his glance returned to the dials, one of them showed immense acceleration. By and by came another triple clanging, another pair of levers was pulled down, and instantly the jarring and clanking gave way to a decided rumble, low and distinct, but so powerful that it shook the air. At the same time the agent quit his post and went over to the giant horizontal cylinder. Now Smith could see that this vast structure was merely part of an engine whose dimensions were quite beyond any former experience. It was a simple affair, being merely a reciprocal machine like the most elementary form of steam engine. But, instead of being operated by steam, it was a chemical machine; Smith's trained eyes told him that the cylinder was really an enormous retort. And he noted with further perplexity that the prodigious piston-rod not only moved with terrific speed, but in a strictly back-and-forth motion; its far end did not revolve. The agent seemed satisfied with it all. He turned about and walked--so far as Smith could sense in the usual manner of earth's humans--back to the dials again. Just then a door opened a short distance away and another man entered. Smith would have mistaken him for the employee of some garage. He was dressed in a suit of greasy blue overalls; and as he advanced toward the eyes Smith was using, he looked about the room with practiced glance. He merely nodded to Smith's man, who returned the nod just as silently; and such was the extreme brevity of it all, Smith was afterward unable to describe the man. His agent, thus relieved of his duty temporarily, strolled out another door, which took him through a narrow corridor and another door, opening on to some sort of a balcony, or deck. Smith fully expected to look upon an ocean. Instead, he found himself gazing into a sea of clouds. He was in some sort of aircraft!