as of importance, for
there would be a leakage and shrinking of the vapor from the big bag,
and some means must be provided for replenishing it.

"You don't seem to have forgotten anything, Tom," said Ned admiringly,
as they soared upward.

"We can tell better after we've flown about a bit," observed the young
inventor, with a smile. "I expect we shall have to make quite a number
of changes."

"Are you going far?" asked Mr. Damon.

"Why, you're not frightened, are you?" inquired Tom. "You have been up
in airships with me before."

"Oh, no, I'm not frightened!" exclaimed the odd man. "Bless my
suspenders, no! But I promised my wife I'd be back this evening, and..."

"We'll sail over toward Waterford," broke in Tom, "and I'll drop you
down in your front yard."

"No, don't do that! Don't! I beg of you!" cried Mr. Damon. "You
see--er--Tom, my wife doesn't like me to make these trips. Of course, I
understand there is no danger, and I like them. But it's just as well
not to make her worry-you understand!"

"Oh, all right," replied Tom, with a laugh. "Well, we're not going far
on this trip. What I want to do, most of all, is to test the guns, and
see if the recoil check will work as well when we are aloft as it did
down on the ground. You know a balloon isn't a very stable base for a
gun, even one of light caliber."

"No, it certainly is not," agreed Lieutenant Marbury, "and I am
interested in seeing how you will overcome the recoil."

"We'll have a test soon," announced Tom.

Meanwhile the Mars, having reached a considerable height, being up so
far, in fact, that the village of Shopton could scarcely be
distinguished, Tom set the signal that told the engine-room force to
start the propellers. This would send them ahead.

Some of Tom's most trusted workmen formed the operating crew, the young
inventor taking charge of the pilot-house himself.

"Well she seems to run all right," observed Lieutenant Marbury, as the
big craft surged ahead just below a stratum of white, fleecy clouds.

"Yes, but not as fast as I'd like to see her go," Tom replied. "Of
course the machinery is new, and it will take some little time for it
to wear down smooth. I'll speed her up a little now."

They had been running for perhaps ten minutes when Tom shoved over the
hand of an indicator that communicated with the engine-room from the
pilot-house. At once the Mars increased her speed.

"She can do it!" cried Ned.

"Bless my-hat! I