Did they hear a gentle hum from high up overhead? Dick fancied he could for one brief instant as the lift shot upward. But it may have been merely imagination. In any case there came quickly enough other sounds to drown any there may have been from the airship; for a monstrous gun spoke in the distance. The air above this devoted city shook and vibrated, while the steel monster launched into space howled and shrieked as it rushed to its destination. "Down behind this wall," called the Commander, who had stood up to stare in the direction from which the shot had come. "Down, quick! That shell's coming straight for us." Throwing themselves down upon the ground behind a low wall beside which the lift had dropped them, they waited breathlessly for the landing of that messenger. It shrieked a warning at them. It announced its coming in a manner there was no mistaking. Then suddenly it burst upon them. The shriek grew positively deafening, rising to such a blood-curdling pitch that it would have shaken the pluck even of a veteran. But it was muffled all in a second. There was a ponderous thud within a dozen yards of the adventurous trio, an uncanny silence, and then a detonation that threw them against the wall, and sent earth and stones and debris in every direction. And what a sight the wide-spreading flames of that explosion presented! Dick saw buildings all about him, buildings over which stones and clods of earth were hurtling. To his left, within two hundred yards perhaps, was an enormous erection, the actual size of which he could not hope to estimate. But the momentary flash of the explosion showed him towers and minarets, proof positive that here was a mosque, the mosque, no doubt, for which Major Harvey had aimed when descending into the city. That fleeting flash gave him in addition just one glimpse of a huge shape floating almost directly overhead, no doubt the gigantic outline of the airship. "Lor! Supposing she felt the shock?" he groaned. "Supposing the airship has sustained some damage." "Not she! As right as a trivet," came in somewhat shaking tones from close beside him, for unconsciously the young midshipman had spoken aloud. "But, jingo, what an explosion! I've been hanging on to my hair ever since. Hope we don't get another of those gentlemen within such close distance." The hope was hardly expressed when a second shell announced its coming, and caused them once more to shelter close to the wall which had already given them protection. As for the giant airship, when Dick gazed aloft as this other messenger exploded, there was no longer a sign of her. No doubt Andrew and his nephew had set the elevators going, and were now high overhead, out of reach of danger. "And so we've only ourselves to think about," said Dick. "What next, then? What are the orders, sir? I caught a sight of the great mosque for which Major Harvey said he would make. It's close to us. I suppose that's where we shall begin our search?"