and Tom had figured just how much force could
be expected to be taken up by a larger check, with stronger hydrostatic
valves, the young inventor explained:

"And now to see how much recoil force my guns develop!"

"Are you really going to fire the guns?" asked Ned.

"Surely," answered Tom. "That's the only way to get at real results.
I'll have the guns taken out and mounted in a big field. Then we'll
fire them, and measure the recoil."

"Well, that may be some fun," spoke Ned, with a grin. "More fun than
all these figures," and he looked at the mass of details on Tom's desk.

This was the second or third day after the fire in the red shed, and in
the interim Tom had been busy making computations. These were about
finished. Meanwhile further investigation bad been made of clues
leading to the origin of the blaze in the shed, but nothing had been
learned.

A photo-telephone had been installed near Eradicate's quarters, in the
hope that the mysterious stranger might keep his promise, and come to
see about the mule. In that case something would have been learned
about him. But, as Tom feared, the man did not appear.

Ned was much interested in the guns, and, a little later, he helped Tom
and Koku mount them in a vacant lot. The giant's strength came in handy
in handling the big parts.

Mr. Swift strolled past, as the guns were being mounted for the
preliminary test, and inquired what his son was doing.

"It will never work, Tom, never!" declared the aged inventor, when
informed. "You can't take up those guns in your air craft, and fire
them with any degree of safety."

"You wait, Dad," laughed Tom. "You haven't yet seen how the Newton
hydrostatic recoil operates."

Ned smiled with pleasure at this.

It took nearly a week to get all the guns mounted, for some of them
required considerable work, and it was also necessary to attach gauges
to them to register the recoil and pressure. In the meanwhile Tom had
been in further communication with government experts who were soon to
call on him to inspect the aerial warship, with a view to purchase.

"When are they coming?" asked Ned, as he and Tom went out one morning
to make the first test of the guns.

"They will be here any day, now. They didn't set any definite date. I
suppose they want to take us unawares, to see that I don't 'frame-up'
any game on them. Well, I'll be ready any time they come. Now, Koku,
bring along those shells, and don't drop any of them,