arrangement of yours--the automatic gas machine."

"It's on the same principle as the air brakes of a trolley car,"
explained Tom, when a look at the indicators showed that the Mars had
ceased falling and remained stationary in the air. Tom had also sent a
signal to the engine-room to shut off the power, so that the two
undamaged propellers, as well as the broken one, ceased revolving.

"In a trolley car, you see," Tom went on, when the excitement had
calmed down, "as soon as the air pressure in the tanks gets below a
certain point, caused by using the air for a number of applications of
the brakes, it lets a magnetized bar fall, and this establishes an
electrical connection, starting the air pump. The pump forces more air
into the tanks until the pressure is enough to throw the pump switch
out of connection, when the pump stops. I use the same thing here."

"And very clever it is," said Mr. Damon. "Do you suppose the danger is
all over, Tom?"

"For the time being, yes. But we must unship that damaged propeller,
and go on with the two."

The necessary orders were given, and several men from the engine-room
at once began the removal of the damaged blades.

As several spare ones were carried aboard one could be put on in place
of the broken one, had this been desired. But Tom thought the accident
a good chance to see how his craft would act with only two-thirds of
her motive force available, so he did not order the damaged propeller
replaced. When it was lowered to the deck it was carefully examined.

"What made it break?" Ned wanted to know.

"That's a question I can't answer," Tom replied. "There may have been a
defect in the wood, but I had it all carefully examined before I used
it."

The propeller was one of the "built-up" type, with alternate layers of
ash and mahogany, but some powerful force had torn and twisted the
blades. The wood was splintered and split, and some jagged pieces,
flying off at a tangent, so great was the centrifugal force, had torn
holes in the strong gas bag.

"Did something hit it; or did it hit something?" asked Ned as he saw
Tom carefully examining the broken blades.

"Hard to say. I'll have a good look at this when we get back. Just now
I want to finish that gun test we didn't get a chance to start."

"You don't mean to say you're going to keep on, and with the balloon
damaged; are you?" cried Mr. Damon, in surprise.

"Certainly--why not?" Tom replied. "In warfare acciden