Henri, mystified, obeyed. "Why?" he asked. "Because when we want to start, we may have to do it in an awful hurry," said Frank. He searched the road for a moment. "Run her back a few feet to where that big tree is. It's darker there than anywhere else around here. All right, that's far enough. We'll have to take the chance of something coming along while we're gone and bumping into her but I don't believe there's much risk of that. Now, come on! And quiet! We've got to get up to that place without being seen." Cautiously they approached the house. No lights showed in any of its windows; the place looked deserted. Indeed, all around it were traces of hasty flight. It was a wayside inn, of a type common always in France, commoner than ever since the spread of the craze for automobiles and motor touring. Suddenly Frank stopped. "Wait a minute for me," he said. "I've got to go back to the car. I ought to have thought of it before." "What do you want?" "Batteries. I saw a coil of wire in the car and I want that, too. And there must be batteries. A car like this would carry everything needed for small repairs, wouldn't it?" "Yes. I think you'll find them under my seat." Frank was back in less than five minutes. "All right," he said. "I don't know whether we'll have time to do what I want or not, and whether I'll be able to do it, anyhow. But it's worth trying. Now come on past the house. Easy! This is the hardest part of it." They slipped by. However, Frank uttered a suppressed exclamation as soon as they had done so. Before them, on the right of the road was a field easily two or three times as large as the ordinary French field. As a rule the land in France is split up into very small sections, closely cultivated. But here was a cleared field as large as those commonly seen in England or America, with no fences for perhaps a quarter of a mile in any direction. Henri turned to look back at the inn. "They're still signalling from there--and look! There are two lights now, instead of one, above!" These lights were still some distance away. Frank studied them. Then he led the way into the field. "I thought so!" he said, with suppressed triumph in his voice. "Do you see those barrels over there toward the inn? There's petrol in those--or I'll eat my shirt!" "And if there is?" said Henri. "What then?" "Can't you guess? What do you suppose those lights mean?" "Aeroplanes?" "Never! They wouldn't flash that way. They'd have to be in a different position entirely. No. Dirigibles!" "Zeppelins?" "Perhaps. Perhaps Parsevals or Schutte-Lanz airships. I think Parsevals, for they need gasoline. And Zeppelins could fly from Brussels or Liege, almost from Cologne--oh, I have it! That's why they need petrol!" "Why?" "They haven't flown over Belgium at all! They are from the sea!" "Oh--so that they could come secretly, and not be seen as they passed over Belgium?"