With a clicking and sliding sound the parts that composed the balancing power of the machine slipped into their places and it resumed an even keel. As if to show his perfect mastery of the military dirigible, Lieut. Diaz drove it straight up toward the overcast sky. A fairly stiff wind was now beginning to blow, and to Jack the maneuver appeared risky in the extreme. But, of course, he said nothing, although, looking downward, earth looked fearfully remote. But to the two Mexican officers all this was evidently part of the day’s work. At all events, neither of them displayed the least anxiety; on the contrary, Lieut. Sancho was busy noting the action of the barograph and barometer, and jotting down the results of his observations in a small notebook. All at once Jack, on glancing down, discovered that the earth had been obliterated. A yellow fog, or it seemed to be fog, hid the surface of the country from them. All at once something stinging struck the boy’s face. It was sand. With a gasp of alarm Jack realized that a sandstorm was raging below them. He recalled the one near La Hacheta, in which the lads had seen the flight of the ghostly camels. Seriously alarmed, he drew the attention of his companions to what was going on. By this time, so rapidly had the velocity of the wind increased that it was blowing half a gale, great clouds of sand swept bewilderingly round them. The aeroplane pitched and swayed like a ship in an angry sea. Jack held on tight, thinking that every moment would be likely to be his last. “We did wrong to come up so high,” admitted Lieut. Diaz. “But you are going higher?” objected Jack. “Yes. We must avoid that sand at all hazards. It won’t be so bad higher up, I hope.” “Why not drop to earth right now? It’s all flat country hereabouts,” said Jack. “In the first place, the sand would blind us and we would crash to earth and be wrecked, in all probability. In the second place, if even a little sand got into our engine it would ruin it,” rejoined the officer. Jack said no more. He felt rather ashamed, in fact, of having showed his agitation so plainly. After all, the officers knew far more about aeroplanes than he did, and perhaps there was a chance that they would get through safely yet, He fervently prayed that they might. Lieut. Diaz sat grimly at the wheel, driving the aeroplane ever upward. Jack watched him admiringly. Not a trace of fear or of any other emotion had flickered across his steadfast countenance since the battle with the storm had begun. They had driven their way far above the yellow sand fog and were battling with the wind at an altitude of almost seven thousand feet, when Lieut. Diaz gave a sudden gasp. He turned deadly pale and lurched forward in his seat. Had not Lieut. Sancho caught him, he would have toppled off into space. The aeroplane, released from a controlling hand, gave a sickening dash downward. “Wha–what has happened?” gasped Jack, genuinely alarmed now.