Far down beneath them now the forest sped away to southward. The gleam of the river, dulled by the sunless sky, showed here and there through the woods, which spread their unbroken carpet to the horizon, impenetrable and filled with nameless perils. At thought of how he was cheating them all, Stern smiled to himself with grim satisfaction. "Good old engine!" he was thinking, as he let her out another notch. "Some day I'll put you in a boat, and we'll go cruising. With you, there's no limit to the possibilities. The world is really ours now, with your help!" Behind them now lay the debris of Pawtucket. Stern caught a glimpse of a ruined building, a crumpled-in gas-tank with an elm growing up through the stark ribs of it, a jumble of wreckage, all small and toylike, there below; then the plane swooped onward, and all lay deep buried in the wilderness again. "A few minutes now," he said to himself, "and we'll be across what used to be the line, and be spinning over Massachusetts. This certainly beats walking all hollow! Whew!" as the machine lurched forward and took an ugly drop. He jerked the rising-plane lever savagely. "Still the same kind of unreliable air, I see, that we used to have a thousand years ago!" For a few minutes the biplane hummed on and on in long rising and falling slants, like a swallow skimming the surface of a lake. The even staccato of the exhaust, echoless in that height and vacancy, rippled with cadences like a monster mowing-machine. And Stern was beginning to consider himself as good as in Boston already--was beginning to wonder where the best place might be to land, whether along the shore or on the Common, where, perhaps, some open space still remained--when another formidable air-pocket dropped him with sickening speed. He righted the plane with a wrench that made her creak and tremble. "I've got to take a higher level, or a lower," he thought. "Something's wrong here, that's certain!" But as he shot the biplane sharply upward, hoping to find a calmer lane, a glance at the sky showed trouble impending. Over the gray background of wind-clouds, a fine-shredded drive was beginning to scud. The whole east had grown black. Only far off to westward did a little patch of dull blue show; and even this was closing up with singular rapidity. And, though the motion of the machine made this hard to estimate, Stern thought to see by the lateral drift of the country below, that they were being carried westward by what--to judge from the agitation of the tree-tops far below--must already be a considerable gale. For a moment the engineer cursed his foolhardiness in having started in face of such a storm as now every moment threatened to break upon them.