rful glasses.
Nothing was seen and the work of loading the guns was begun. The bomb
tubes, in the main cabin, were also to be given a test.

As service charges were to be used, and as the projectiles were filled
with explosives, great care was needed in handling them.

"We'll try dropping bombs first," Tom suggested. "We know they will
work, and that will be so much out of the way."

To make the test a severe one, small floating targets were first
dropped overboard from the Mars. Then the aerial warship, circling
about, came on toward them. Tom, seated at the range-finders, pressed
the button that released the shells containing the explosives. One
after another they dropped into the sea, exploding as they fell, and
sending up a great column of salt water.

"Every one a hit!" reported Lieutenant Marbury, who was keeping "score."

"That's good," responded Tom. "But the others won't be so easy. We
have nothing to shoot at."

They had to fire the other guns without targets at which to aim. But,
after all, it was the absence of recoil they wanted to establish, and
this could be done without shooting at any particular object.

One after another the guns were loaded. As has been explained, they
were now made double, one barrel carrying the projectile, and the other
a charge of water.

"Are you ready?" asked Tom, when it was time to fire. Lieutenant
Marbury, Ned and Mr. Damon were helping, by being stationed at the
pressure gauges to note the results.

"All ready," answered Ned.

"Do you think we'd better put on life preservers, Tom?" asked Mr. Damon.

"Nonsense! What for?"

"In case--in case anything happens."

"Nothing will happen. Look out now, I'm going to fire."

The guns were to be fired simultaneously by means of an electric
current, when Tom pressed a button.

"Here they go!" exclaimed the young inventor.

There was a moment of waiting, and then came a thundering roar. The
Mars trembled, but she did not shift to either side from an even keel.
From one barrel of the guns shot out the explosive projectiles, and
from the other spurted a jet of water, sent out by a charge of powder,
equal in weight to that which forced out the shot.

As the projectile was fired in one direction, and the water in one
directly opposite, the two discharges neutralized one another.

Out flew the pointed steel shells, to fall harmlessly into the sea,
where they exploded, sending up columns of water.

"Well!" cried Tom