recoil
business. Maybe you can help me."

"Fire away," laughed Ned. "I'll do the best I can."

Tom led the way from the main shed, where the aerial warship was
housed, to a small private office. As Ned entered, the door, pulled by
a strong spring, swung after him. He held back his hand to prevent it
from slamming, but there was no need, for a patent arrangement took up
all the force, and the door closed gently. Ned looked around, not much
surprised, for the same sort of door-check was in use at his bank. But
a sudden idea came to him.

"There you are, Tom!" he cried. "Why not take up the recoil of the guns
on your aerial warship by some such device as that?" and Ned pointed to
the door-check.

CHAPTER VII

WARNINGS

For a moment or two Tom Swift did not seem to comprehend what Ned had
said. He remained staring, first at his chum, who stood pointing, and
from him Tom's gaze wandered to the top of the door. It may have been,
and probably was, that Tom was thinking of other matters at that
instant. But Ned said again:

"Wouldn't that do, Tom? Check the recoil of the gun with whatever stuff
is in that arrangement!"

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."

"And do you think you can put something like that on your guns, so the
recoil will be taken up?" Ned wanted to know.

"I think so," spoke Tom. "I'm going to work on it right away, and we'll
soon see how it will