But the main deck guns could be elevated to an angle of nearly forty-five degrees, so they could take care of nearly any hostile aircraft that approached.

“But where are the bombs I heard you speaking of?” asked Ned, as they finished looking at the guns.

“Here they are,” spoke Tom, as he pointed to a space in the middle of the main cabin floor.  He lifted a brass plate, and disclosed three holes, covered with a strong wire netting that could be removed.  “The bombs will be dropped through those holes,” explained the young inventor, “being released by a magnetic control 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.