He grinned a wicked grin at Dick and went racing away from him. As for the young sailor, he gave chase on the instant, so that presently the ship rang with their merry cries. And indeed, they made a race of it, for Alec made for the gangway built around the lift, racing down the steep stairs as fast as active legs could carry him. Dick, however, proved his salt and his training. Finding a smooth, central girder of that strange and transparent material, he wrapped his legs round it, and went shooting like a descending rocket to the deck below where he arrived with a resounding bump, to find himself directly facing Commander Jackson.

"'Mornin', sir!" he gasped, drawing himself up and touching the peak of his hat. "Fine weather, sir."

"For monkey tricks, yes," laughed the Commander. "Well, lad, how do you like the vessel? Seen the aeroplane? Eh? Like a trip aboard her? I'm the coxswain."

"Rather, sir," gasped Dick. "This is the finest thing I've had to do since I joined the Navy."

"Indeed! You've been an officer a long while I take it," smiled his senior. "Quite one of the older ones, Dick, eh? Come; I'll stop quizzing. Let's get along to the engines; Joe Gresson has gone there. There's no keeping him away from them. Come; you'll see the height of simplicity combined with the uttermost efficiency that has yet been attained."

Dick did indeed inspect a machine which, with its components, gave extraordinary power to the ship. To put the description with the utmost plainness, he found when he descended to the engine-room three sets of engines, of moderate size, and of the internal-combustion variety. There was nothing remarkable, perhaps, about the engines themselves, except that they were a modification of the Diesel.

"You see, a Diesel uses extremely high compression," Joe Gresson explained, leaning one hand affectionately on an engine which happened not to be working. "That can be managed easily ashore, and in the air also. But compressors are required in addition to the engine, for the explosive charge, consisting of the crudest oil, must be injected into the cylinders by pneumatic power at a critical moment, and that power must be at higher pressure than the contents of the cylinder. To me the most important question was the one of fuel. I barred petrol."

"Why?" Dick ventured to ask. "It's used on other airships."

"And other ships suffer from explosions and from fire. Petrol is too inflammable, particularly upon a ship which is lifted by a huge volume of gas. So I chose crude paraffin oil, the sort of oil that you can obtain in any part of Europe, almost in any part of the world. To discover a carburettor which would vaporize this crude oil was difficult. But a friend came to my help, and here you see the result. Our engines run steadily and strongly."

He pointed to the other two, which, as he said, were turning over noiselessly and with a rhythm that told its tale plainly. Even Dick had sufficient experience of this class of engine to know that the running was excellent. But beyond that he was somewhat fogged. For besides some machinery housed in at the end of each motor, and a certain number of switches and levers common to any engine-room, there was nothing to indicate in what manner the power of the engines was conveyed, nor in what direction. Where was the propeller? How did these motors operate it? By electricity? Perhaps, for he could see a large dynamo revolving at the far end of the cabin. But he was by no means certain. He asked the question instantly, causing Joe to raise his head, open a port at the far end of the cabin, and invite him to look through it.

"We're a little aft of amidships here," he explained, "and form the lowest attachment to the vessel. We're dead in the central line, and the weight of these motors and of other accessories housed in what compares with the keel of an ordinary ship, keeps her perfectly steady. Now, look yonder. That is the tail end of the ship. You can see the propeller, and as it is revolving and you cannot, therefore, distinguish its outline I had better tell you something about it. To begin, it's both propeller and rudder. See, I wish to turn the vessel. I press this lever to the right and at once the propeller swings in the same way, driving the tail of the ship to the left. See, I reverse the motion. Or, perhaps, I wish to descend or rise – hold tight, please, gentlemen, while I give our friend here a little demonstration! But first, let me say that the propeller itself is forty feet in diameter, presents half a dozen blades, the pitch of which can be instantly altered, while the blades are encircled by a tube some twenty feet in depth from back to front. Thus the air drawn into this revolving tube cannot escape to either side, and the blades lose no efficiency, while one can readily understand that when the ship is travelling quickly, particularly against a head wind, the alteration in the pitch of the blades makes for greater speed and more effectual use of the power. Now, hold tight, please. We'll show our friend of what we are capable."

At a touch upon a lever the propeller that Dick was watching, and which was rotating very slowly, suddenly gathered speed, till it was but a mere haze in the distance. He felt the whole ship move forward, while a touch on another lever bent the propeller downward, and to his consternation the deck he stood on canted badly, the vessel headed downward and went hurtling towards that muddy ocean which he could see below him. The sensation was in fact paralysing. It was worse, perhaps, when it was reversed, and the nose of the ship shot upward, setting the deck at such an angle that Dick had to cling hard to the railings fending the motors. But a moment later, at a touch from the inventor, she came to an even keel, the propeller ceased to rotate, while the vessel came to a halt.

"Now, see how we rise at will," said Joe, watching Dick's face with delight, for it pleased the young inventor to notice the open admiration with which the youthful sailor regarded everything. "Now, I pull this handle. We fall. I reverse the movement. We shoot upward, but always keeping the horizontal position."

It was really remarkable, for the mere touch of the inventor sent the ship up and down, for all the world as if she were suspended in space, and his fingers controlled the switch of some hoisting machinery.

"How's it done?" asked Dick eagerly. "How does the power get to that propeller, for instance? Your motors are here. There are no chains, no shafts, nothing save these cased-in things at the end of each motor, which might be pumps for all I know."

"And happen to be exactly what you have mentioned. They are pumps, of the rotary variety, and the material they deal with is that same common, crude paraffin on which our motors run. See those pipes. They are of the best, cold-drawn steel. They convey the oil from our pumps to the various propellers, to the lift and to any part where we have need for power. No corner is too sharp for them. They run anywhere, and, as you can imagine, convey the power of these engines with a certainty there is no gainsaying. Of course, at the far end we have other rotary motors. The oil pumped at this point, and under high pressure, is unable to escape from the steel pipes. At will we pass it into our distant motors, allowing some to escape back in this direction through a bypass. If the bypass is pulled wide open, the motors beyond do not turn; for the oil fails to reach them. If it is closed, there is no escape for the oil. It reaches the motors at its highest pressure, and operates them at full power, as powerfully, in fact, as if this engine down here driving the pumps was away up there close to the propeller with the shaft directly coupled to it. In short, and as an interesting fact, our propellers and other gear are operated by hydraulic power, applied after the latest principle."

"Which is a lesson that will keep you for a while," smiled the Commander. "Ah, there's our host, and I hear the breakfast gong sounding. Come, Dick, my lad, you could eat, you think? the great height at which we fly does not rob you of your appetite?"

Not by a long way. The young fellow was beginning to revel in his strange surroundings, and to quite like this residence at a height. More than all, he was vastly interested in the intricacies of the vessel. And we record only the fact when we say that he and Alec spent the whole of the morning in a close and thorough investigation, an investigation which disclosed, among other matters, the interesting fact that centrally-placed propellers, operating in tubes built transversely at either end of the ship, controlled her sideways movements, making entrance to a hangar easy, while she could be caused to descend or rise by others, located fore and aft likewise, with their tubes built in the vertical direction. As for the huge framework that held the gas, it was divided into twenty compartments, to each of which pipes of that strange transparent material led. These latter ended in one large branch which was attached to a machine at one end of the engine-room. Joe explained its action with a gusto that showed it to be one of his pet items, one on which he prided himself not a little.

"What's the good of a ship which has to constantly return to land for gas supplies?" he said. "We take ours with us. Not compressed and in steel cylinders. Anyone can do that. But in the form of fuel. We carry a matter of seven tons, and can get a further supply in any part of the world. A gas producer of my own designing deals with the stuff, and at desire we can supply gas to replace leakages. Not that we experience those. Otherwise it would not be safe to smoke. But each one of our compartments aboard the ship contains a proportion of air. When we want to go higher, or lift a bigger weight, we simply set the producer going, and by means of one of our motors pump gas to the top of the compartments. Simple, isn't it? But it makes us wonderfully independent. That's why we've undertaken to make a trip round the world."

"Round the world? When?"

"Now – to-morrow, that is to say."

"And – and I go with you?" gasped Dick.

"Of course; you've been detailed by the Admiralty."

The shout which the midshipman gave might have been heard at the far end of the vessel. The prospect filled him with delight, so that he was simply boiling over with enthusiasm and anticipation. Nor did his excitement evaporate as the day advanced. For the ship manœuvred over the ocean, and was put through her paces. Towards evening, however, her nose was turned to the north-east, and as night fell she hovered over England. Slowly she descended, obscured in darkness, till her pilot was able to pick up his bearings. A distant light of curious colour caught his eye and he sent the ship towards it. Then, when directly overhead that same brilliant light suddenly shot from the ship and flooded the buildings beneath. Dick found himself looking down upon a huge shed, placed in a wide-open place, and – could he believe his eyes? – the shed was moving, revolving, actually turning. It made him giddy. Or was it this wonderful ship which was turning?

"It's the revolving hangar, of course," Alec told him, with a laugh. "You see, if the wind's blowing, we head up to it. The hangar opens away from the point from which the gale comes. We manœuvre opposite it, and enter easily. You watch. No one is wanted to hang on to ropes. Our pilot can manage the ship in any direction. See, we've dropped opposite the shed; but we're not quite head on. We're getting near it, however, for those propellers located in the cross tubes are being set to work. Ah! that's better. See, we're creeping in. Now our huge lateral keels run into wide slots built into the sides of the hangar. They engage and run in farther. Right! We're home. Welcome to our kennel."

And what of the trip promised by Joe Gresson, and of the adventures it might and certainly should bring in its train?

"Jingo!" cried Dick, as he hastened to the post office to send a telegram requesting that his full kit might be sent to him. "Jingo, if we don't have a cruise worth talking about, well – well, my name ain't Dicky."