“Fall?”
“Yes, but very gradually at first, so long as our speed was fairly well maintained. The aëroplane, as you know, will only buoy us up on the condition that we move, and that pretty quickly. Still, there are always the two spare steering screws to fall back upon.”
“But what if they stopped as well?”
“It’s most unlikely that they would stop. The three shafts are worked independently. But if they did, the sand-valves would have to be opened.”
“The sand-valves?”
“Yes. You have doubtless been surprised at the huge size of the Attila. Well the main parts of the upper and middle portions of her hull are nothing more nor less than a succession of gas-meters—of compartments filled with hydrogen introduced at a high temperature, so as to yield the maximum amount of buoyancy. Below these compartments again lie the sand reservoirs. When these latter are three parts full their natural effect is to keep the Attila at about the level of the sea, supposing, that is to say, the screws are completely stopped. If your so-much-dreaded event was to happen, the watch in the conning-tower would simply shift the sand-levers, a quantity of ballast would be released, and we should at once begin to rise. We can thus regulate our weight at will. The secret of it all is the marvellous lightness of these walls. I am not free to tell you to what discovery that lightness is due, but you may test and analyze as much as you like, on the off chance of a correct guess.”
“It’s all superb!” was my enthusiastic comment. “But how about an ordinary complete descent to earth?”
“A very simple matter. From the outer gallery the Attila looks as if her bottom was gently curved, terminating in the customary orthodox keel. That is what the upper lines suggest. But three feet below the level at which we stand lies a flat projecting bottom studded underneath with springs resting on the axles of wheels. I wish to touch land. I press certain knobs and this, that, perhaps all three screws, ease off, run down, or may be reversed. The Attila then sweeps onward much after the fashion of an albatross with outspread motionless wings. Steering is easy—a ‘ting’ in the engine-room sets this or that side screw shaft rotating. Slowly—perhaps fast—she falls, then faster and faster. Meanwhile I stand by the sand-levers—I pull this and the stern rises, we swoop down like a hawk; I pull that, the bow rises, the impetus thus gained carries the Attila in a noble curve aloft. Finally she hovers over the ground, and, opening a hydrogen valve, I adjust her descent delicately, so as to spare the springs.”
“But you must lose a great deal of hydrogen in this manner.”
