"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."