"I expect it’s the petrol," said Bob humbly. "You didn’t mention petrol." "No; but of course we couldn’t work the engine without it, and I left word to send up a good quantity. I didn’t suppose you had any on the spot." "And wasn’t there a single sensible creature to tell you that you can’t go skylarking with an aeroplane in the Hindu Kush? Whoever sold you the petrol must have laughed in his sleeve." "He seemed uncommon glad to sell it, anyway," said Bob, a trifle nettled. "Of course he was. There are no end of sharks always on the watch for a griffin. He sold the petrol, and he sold you. And the expense of it! D’you know how much it costs to bring a mule from India here?" "You can dock it out of my screw," growled Bob. "And money absolutely flung away. You have seen for yourself that there’s no level space hereabout for running off. And even supposing you could use the thing, it would be madness to do so. You’d be bound to come to grief; all flying men do sooner or later, and at the best you might find yourself landed thirty or forty miles away, with nothing but peaks and precipices between you and home. There are no repairing shops to fall back upon; no garages ’open day and night,’ or anything of that sort. In short----" "Don’t rub it in, Uncle," said Lawrence. "The thing’s here now, and we’ve got to make the best of it. Come on, Bob; let’s go and look after the unloading; those fellows are sure to smash something." The mules were led across the drawbridge to the west side of the gorge, and the separate parts of the machine were stacked near the dwelling house until a new shed could be constructed. "What on earth we’re to do with the petrol I don’t know," said Mr. Appleton. "We daren’t have it within reach of the native workmen. They’re as careless as they are inquisitive, and we don’t want a flare up." "Isn’t there room for the cans in the dynamite shed?" asked Lawrence. The explosive was kept in a specially devised cache. The space between the house and the cliff was boarded in. A doorway led from the house into this space, which was divided by a partition, in which another door opened into a kind of strong room excavated in the hill side. There was room for the cans beside the boxes of dynamite. "I shan’t sleep at night now that we’ve got two explosives at our doors," said Mr. Appleton. "Why didn’t you store the stuff farther from the house?" asked Bob. "Well, as a matter of fact wherever it was stored in the neighbourhood of the mine the result would be pretty much the same if it exploded. The best chance of safety was to have it under lock and key where nobody could get at it but myself. In for a penny, in for a pound. Trundle your cans through: if I’m not a false prophet they’ll stay there until doomsday untouched." When the boys entered the dark chamber between the house and the cliff, following Mr. Appleton, who carried an electric lamp, Bob uttered a sudden exclamation.