l when the operator
thinks he has reached a spot over the enemy's city or fortification
where the most damage will be done. I'll show you how they work a
little later. Now we'll have a test of some of the guns."

Tom called for some of his men to take charge of the steering and
running of the Mars while he and Lieutenant Marbury prepared to fire
the two larger weapons. This was to be one of the most important tests.

Service charges had been put in, though, of course, no projectiles
would be used, since they were then flying over a large city not far
from Shopton.

"We'll have to wait until we get out over the ocean to give a complete
test, with a bursting shell," Tom said.

He and Lieutenant Marbury were beside a gun, and were about to fire it,
when suddenly, from the stern of the ship, came a ripping, tearing
sound, and, at the same time, confused shouts came from the crew's
quarters.

"What is it?" cried Tom.

"One of the propellers!" was the answer. "It's split, and has torn a
big hole in the gas bag!"

"Bless my overshoes!" cried Mr. Damon. "We're going down!"

All on board the Mars became aware of a sudden sinking sensation.

CHAPTER XVI

TOM IS WORRIED

"Steady, all!" came in even tones from Tom Swift. Not for an instant
had he lost his composure. For it was an accident, that much was
certain, and one that might endanger the lives of all on board.

Above the noise of the machinery in the motor room could be heard the
thrashing and banging of the broken or loose propeller-blade. Just
what its condition was, could not be told, as a bulge of the gas bag
hid it from the view of those gathered about the gun, which was about
to be fired when the alarm was given.

"We're sinking!" cried Mr. Damon. "We're going down, Tom!"

"That's nothing," was the cool answer. "It is only for a moment. Only a
few of the gas compartments can be torn. There will soon enough
additional gas in the others to lift us again."

And so it proved. The moment the pressure of the lifting gas in the big
oiled silk and aluminum container was lowered, it started the
generating machine, and enough extra gas was pumped into the uninjured
compartments to compensate for the loss.

"We're not falling so fast now," observed Ned.

"No, and we'll soon stop falling altogether," calmly declared Tom. "Too
bad this accident had to happen, though."

"It might have been much worse, my boy!" exclaimed the lieutenant.
"That's a great