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."