"Rather," agreed Sir Reginald. "Sufficient to lift her independently of the gas-bag, while in the unlikely event of the motors giving out there is enough lifting power in the envelope to keep her up for an indefinite period. Did you notice the small propellers in the wake of the large ones?" "Yes, sir," replied Bramsdean. "Left-handed blades." "Precisely," agreed Fosterdyke. "They work on the same shaft, only in a reverse direction. It’s a little stunt of mine to utilise the eddies in the wake of the main propellers. Yes, petrol-driven. I tried to find an ideal fuel, one that is non-inflammable or practically so, except in compression; but that’s done me so far. There’s a huge fortune awaiting the chemist who succeeds in producing a liquid capable of conforming to these conditions. I even made a cordite-fired motor once--something on the Maxim-gun principle, fed by cordite grains from a hopper. It did splendidly as far as developing power was concerned, but the difficulty of excessive consumption and the pitting of the walls of the cylinder did me. However, my experiments haven’t all been failures. Now look at the gas-bag." "It’s only partly inflated," observed Peter. "No, fully," corrected Fosterdyke. "The envelope is a rigid one of aluminium, subdivided into forty-nine compartments, each of which contains a flexible ballonet. Each ballonet is theoretically proof against leakage--in practice there is an almost inappreciable porosity, which hardly counts for a comparatively short period, say a month. The gas isn’t hydrogen, nor is it the helium we used during the war. Helium, although practically non-inflammable, is heavier than hydrogen. Fortunately, I hit upon a rather smart youngster who had been in a Government laboratory before he joined the R.A.F. With his assistance I discovered a gas that is not only lighter than hydrogen, but is as non-inflammable as helium. I’ve named the stuff ’Brodium,’ after the youngster who helped me so efficaciously. When this stunt’s over, we’re going to work the gas on a commercial basis, but for the present it’s advisable to keep it a secret. "You observe that the section of the envelope is far from being circular. The horizontal diameter is three-and-a-half times that of the vertical. That gives less surface for a side wind, and consequently less drift, while the ’cod’s head and mackerel tail’ ought to give a perfect stream-line." "You carry a pretty stiff lot of fuel with those motors," remarked Kenyon.