The aëroplane flew very high at first--so high that the two men in it appeared the merest specks, but almost at once two high-angle guns were banging away at it, though the shells fell far short. Gradually it circled lower and lower, as if quite unconscious of the marksmen in the valley, and as it swept past the hill, Stewart glimpsed the men quite plainly--one with his hands upon the levers, the other, with a pair of glasses to his eyes, eagerly scanning the ground beneath. And then Stewart, happening to glance toward the horizon, was held enthralled by a new spectacle. High over the hills to the east flew a mammoth shape, straight toward the fort. Its defenders saw their danger instantly, and hastily elevating some of their guns, greeted the Zeppelin with a salvo. But it came straight on with incredible speed, and as it passed above the fort, a terrific explosion shook the mountain to its base. Stewart, staring with bated breath, told himself that that was the end, that not one stone of that great fortress remained upon another; but an instant later, another volley sent after the fleeing airship told that the fort still stood--that the bomb had missed its mark. The aëroplane scouts, their vision shadowed by the broad wings of their machine, had not seen the Zeppelin until the explosion brought them sharp round toward it. Then, with a sudden upward swoop, they leaped forward in pursuit. But nothing could overtake that monster,--it was speeding too fast, it was already far away, and in a moment disappeared over the hills to the west. So, after a moment's breathless flight, the biplane turned, circled slowly above the fort, and dropped down toward the town behind it. Five minutes later, a high-powered shell burst squarely in the midst of the German battery, disabling two of the guns. At once the horses were driven up and the remaining guns whirled away to a new emplacement, while a passing motor ambulance was stopped to pick up the wounded. Stewart, who had been watching all this with something of the feelings of a spectator at some tremendous panorama, was suddenly conscious of a mighty stream of men approaching the river from the head of the valley. A regiment of cavalry rode in front, their long lances giving them an appearance indescribably picturesque; behind them came column after column of infantry, moving like clock-work, their gray uniforms blending so perfectly with the background that it was difficult to tell where the columns began or where they ended. Their passage reminded Stewart of the quiver of heat above a sultry landscape--a vibration of the air scarcely perceptible. All the columns were converging on the river, and looking toward it, Stewart saw that the bridge was almost done. As the last planks were laid, a squadron of Uhlans, which had been held in readiness, dashed across, and deploying fanshape, advanced to reconnoiter the country on the other side. "That looks like invasion in earnest!" said Stewart.