ch happened to be when we took our first flight, the acid
ate through whatever it was contained in, and then attacked the wood of
the propeller blade. It weakened the wood so that the force used in
whirling it around broke it."

"Are you sure of that?" asked Ned.

"As sure as I am that I'm here! Now I know what caused the accident!"

"But who would play such a trick?" asked Ned. "We might all have been
killed."

"Yes, I know we might," said Tom. "It must be the work of some of those
foreign spies whose first plot we nipped in the bud. I must tell
Marbury of this, but don't mention it to dad."

"I won't," promised Ned.

Lieutenant Marbury agreed with Tom that someone had surreptitiously
bored a small hole in the propeller blade, and had inserted a corrosive
acid that would take many hours to operate. The hole had been varnished
over, probably, so it would not show.

"And that means I've got to examine the other two blades," Tom said.
"They may be doctored too."

But they did not prove to be. A careful examination showed nothing
wrong. An effort was made to find out who had tried to destroy the Mars
in midair, but it came to nothing. The two men in custody declared they
knew nothing of it, and there was no way of proving that they did.

Meanwhile, the torn gas bag was repaired, and Tom began working on the
problem of doing away with the gun recoil. He tried several schemes,
and almost was on the point of giving up when suddenly he received a
hint by reading an account of how the recoil was taken care of on some
of the German Zeppelins.

The guns there were made double, with the extra barrel filled with
water or sand, that could be shot out as was the regular charge. As
both barrels were fired at the same time, and in opposite directions,
with the same amount of powder, one neutralized the other, and the
recoil was canceled, the ship remaining steady after fire.

"By Jove! I believe that will do the trick!" cried Tom. "I'm going to
try it."

"Good luck to you!" cried Ned.

It was no easy matter to change all the guns of the Mars, and fit them
with double barrels. But by working day and night shifts Tom managed
it. Meanwhile, a careful watch was kept over the shops. Several new men
applied for work, and some of them were suspicious enough in looks, but
Tom took on no new hands.

Finally the new guns were made, and tried with the Mars held on the
ground. They behaved perfectly, the shooting of sand or water