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 searchlight 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 keep dark until I can
see what's the matter. We must leave here before daylight."

"Suppose we can't?" asked Mr. Damon. "The crowds will be sure to see us
then, anyhow."

"I am pretty sure we can get away," was the opinion of the balloonist.
"Even if our gas container is so damaged that it will not sustain us,
we are still an aeroplane, and this roof being flat will make a good
place to start from. No, we can leave as soon as this storm lets up a
little."

"Then I'm going to have a look and find out what sort of a building
this is," declared Tom, and, while Mr. Sharp began a survey, as well as
he could in the dark, of the airship, the young inventor proceeded
cautiously to ascertain the extent of the roof.

The rain was not coming down quite so hard now, and Tom found it easier
to see. Mr. Damon, finding he could do nothing to help, went back into
the cabin, blessing himself and his various possessions at the queer
predicament in which they found themselves.

Flashing his light every few seconds, Tom walked on until he came to
one edge of the roof. It was very large, as he could judge by the time
it took him to traverse it. There was a low parapet at the edge. He
peered over, and an expanse of dark wall met his eyes.

"Must have come to one side," he reasoned. "I want to get to the front.
Then, maybe, I can see a sign that will tell me what I want to know."

The lad turned to the left, and, presently came to another parapet. It
was higher, and ornamented with terra-cotta bricks. This, evidently,
was the front. As Tom peered over the edge of the little raised ledge,
there flashed out below him hundreds of electric lights. The city
illuminating plant was being repaired. Then Tom saw flashing below him
one of those large signs made of incandescent lights. It was in front
of the building, and as soon as our hero saw the words he knew where
the airship had landed. For what he read, as he leaned over, was this:

MIDDLEVILLE ARCADE

Tom gave a cry.

"What's the matter?" called Mr. Sharp.

"I've discovered something," answered Tom, hurrying up to his friend.
"We're on top of the Middleville Arc