A sudden change came over Tom's face. It was lighted up with a
gleam of understanding.

"By Jove, Ned, old man!" he cried. "I believe you've struck it!
And to think that has been under my nose, or, rather, over my
head, all this while, and I never thought of it. Hurray! That
will solve the problem!"

"Do you think it will?" asked Ned, glad that he had contributed
something, if only an idea, to Tom's aerial warship.

"I'm almost sure it will. I'll give it a trial right away."

"What's in that door-check?" Ned asked. "I never stopped before
to think what useful things they are, though at the bank, with
the big, heavy doors, they are mighty useful."

"They are a combination of springs and hydrostatic valves,"
began Tom.

"Good-night!" laughed Ned. "Excuse the slang, Tom, but what in
the world is a hydrostatic valve?"

"A valve through which liquids pass. In this door-check there
may be a mixture of water, alcohol and glycerine, the alcohol to
prevent freezing in cold weather, and the glycerine to give body
to the mixture so it will not flow through the valves too
freely."