“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, ruefully.  “I’m afraid I’ll have to begin all over again, and proceed along new lines.”

“Well, perhaps you will,” said the lieutenant.  “But you may invent something much better than anything you have now.  There is no great rush.  Take your time, and do something good.”

“Oh, I’ll get busy on it right away,” Tom declared.  “We’ll go down now, and start right to work.  I’m afraid, Ned, that our idea of a door-spring check isn’t going to work.”

“I might have known my idea wouldn’t amount to anything,” said the young bank clerk.

“Oh, the idea is all right,” declared Tom, “but it wants modifying.  There is more power to those recoils than I figured, though our first experiments seemed to warrant us in believing that we had solved the problem.”

“Are you going to try the bomb-dropping device?” asked the lieutenant.

“Yes, there can’t be any recoil from that,” Tom said.  “I’ll drop a few blank ones, and see how accurate the range finders are.”

While his men were getting ready for this test Tom bent over the broken propeller, looking from that to the recoil checks, which had not come up to expectations.  Then he shook his head in a worried and puzzled manner.