"A roof?" repeated Mr. Damon.
"Yes; the roof of some large building, and what you thought was a river is the rain water running off it. See!"
The young inventor held the light down so his companions could observe the surface of that upon which the airship rested. There was no doubt of it. They were on top of a large building.
"If we're on a roof we must be in the midst of a city," objected Mr. Damon. "But I can't see any lights around, and we would see them if we were in a city, you know."
"Maybe the storm put the lights out of business," suggested Mr. Sharp. "That often occurs."
"I know one way we can find out for certain," went on Tom.
"How?"
"Start up our search lamp, and play it all around. We can't make sure how large this roof is in the dark, and it's risky trying to trace the edges by walking around."
"Yes, and it would be risky to start our search light going," objected Mr. Sharp. "People would see it, and there'd be a crowd up here in less than no time, storm or no storm. No, we've got to
