Excuse me!" said Mr. Damon firmly. "I'd rather go down, if it's all
the same to you. If my wife ever knew I was here I'd never hear the
last of it!"

"We'll go down soon," Tom promised. "But I must fire a couple of shots
more. You wouldn't call the recoil checks a success, would you?" and
the young inventor appealed to the government inspector.

"No, I certainly would not," was the prompt answer. "I am sorry, too,
for they seemed to be just what was needed. Of course I understand this
is not an official test, and I am not obliged to make a report of this
trial. But had it been, I should have had to score against you.

"I realize that, and I'm not asking any favors, but I'll try it again
with the recoil checks tightened up. I think the hydrostatic valves
were open too much, also."

Preparations were now made for firing the four-inch guns once more. All
this while the Mars had been speeding around in space, being about two
miles up in the air. Tom's craft was not designed to reach as great an
elevation as would be possible in an aeroplane, since to work havoc to
an enemy's fortifications by means of aerial bombs they do not need to
be dropped from a great height.

In fact, experiments in Germany have shown that bombs falling from a
great height are less effective than those falling from an airship
nearer the earth. For a bomb, falling from a height of two miles,
acquires enough momentum to penetrate far into the earth, so that much
of the resultant explosive force is expended in a downward direction,
and little damage is done to the fortifications. A bomb dropped from a
lower altitude, expending its force on all sides, does much more damage.

On the other hand, in destroying buildings, it has been found desirable
to drop a bomb from a good height so that it may penetrate even a
protected roof, and explode inside.

Once more Tom made ready to fire, this time having given the recoil
checks greater resistance. But though there was less motion imparted to
the airship when the guns were discharged, there was still too much for
comfort, or even safety.

"Well, something's wrong, that's sure," remarked Tom, in rather
disappointed tones as he noted the effect of the second shots. "If we
get as much recoil from the two guns, what would happen if we fired
them all at once?"

"Don't do it! Don't do it, I beg of you!" entreated Mr. Damon. "Bless
my toothbrush--don't do it!"

"I won't--just at present," Tom said, ruefull