“What is it then?” demanded Jerry. “Did some one roll a single bicycle wheel about on the roof for fun?”
Before Ned could reply, Bob, who had gone off several paces to the left, uttered a cry.
“Here’s another!” he shouted, pointing to the dusty tin roof. His companions hastened over, taking care to keep off the tracks, and there saw another mark, exactly like the first.
For a moment Jerry Hopkins stared at the second impression. Then he went back to look at the first one. Next he hurried forward and began looking at a space about midway between the two tire tracks. His companions and the policeman watched him curiously. Suddenly Jerry threw up his hand as a signal.
“I’ve found it!” he cried.
“What?” asked Ned.
“The third track!” was the response. “Fellows it was no bicycle up here. It was——”
“An aeroplane!” fairly burst out Ned and Bob together, for now, with the discovery of the third impression, midway between and ahead of the first two, it was very plain to anyone who had had to do with aeroplanes that they were the marks of the three landing, or starting wheels, of such a craft, that had left the marks in the dust of the roof.
“An airship!” exclaimed the policeman. “Do you boys mean to say that an airship has been up here?”
“It certainly has,” declared Jerry firmly. “Look here! There are more marks farther on.” He pointed just beyond a blank space, where the tin roof was clean of dust, and the marks were again visible in the soft tar of another roof. “They landed here and made a start from here. They could easily do it. In fact this long, flat roof with the tar and gravel to give good traction, is an ideal starting place for an aeroplane.”