“Almost all gone!” he laughed, as he sent the _Hummer_ up at a little steeper angle to gain a higher altitude more quickly. The two young people gave themselves up to the thrill and revivifying influence of clear, pure, sunlit air. Deeply they breathed in of the life-giving particles, and the cheeks of Tom and Mary were ruddy with renewed health. With no special object in view, they spun on through the air, now going up until they were above some low-lying clouds and again dipping down to view with pleasure the contour of some wonderful, green valley. “Getting tired, Mary?” called Tom, after a while. “No!” she called back to him. “I could go on like this forever.” “Guess I’ll have to invent some new kind of machine if you want to do anything like that,” the youth countered. “What do you mean?” challenged Mary. “I mean perpetual motion hasn’t yet been solved, and I don’t believe it ever will be. As long as we have gasoline engines they will have to be given a drink now and then. Which reminds me that I haven’t enough in the tanks of the _Hummer_ to go on for more than a few hours more.” “I don’t want to ride quite that long, of course! Don’t take any chances. Go back now if you think anything is going to happen.” “Nothing is likely to happen!” chuckled the young inventor. “But I didn’t want you to go on dreaming that dream of yours about keeping on forever.” “As if I meant that!” laughed the girl. “Better turn back now.” “In a little while,” promised Tom, whose eyes were just then fixed on some object or some view just ahead. Mary was quick to notice his preoccupation as he spoke to her and at once asked: “Is anything the matter, Tom?” “Matter? No! Why do you ask?” “Because you weren’t thinking of what you were saying, that’s all. I can always tell. Do you see anything?” “To be perfectly frank, Mary, I do. I see a cloud of smoke over there in the direction of Shopton, and when I see smoke I think of fire. As we recently had a little blaze at one of the shops, I am a bit anxious to see if this is another. Of course it will be as well fought with me away as with me there. But still——” “Oh, Tom, I see it, too!” cried the girl, as a little puff of smoke made itself visible near a wooded part of the country. “Perhaps you’d better head back that way.” “I think I will,” decided the pilot. He moved the steering wheel slightly, banked the plane a bit, and was off in another direction, heading directly for the haze of smoke which by this time had considerably increased in volume. At the time when Tom first saw the smoke menace he was several miles from it, though the clear air made the fire seem nearer than it was. But the _Hummer_ was a speedy craft, and she quickly covered the distance.