elf with it, and on
another occasion some of the slow-burning powder, failing to explode,
had set ablaze a shack in which he was working.

Only for the prompt action of Koku, Tom might have been seriously
injured. As it was he lost some valuable patterns and papers.

But he had gone on his way, surmounting failure after failure, until
now he was ready for the supreme test. This was to be the explosion of
a large quantity of the powder in a specially prepared steel tube of
great thickness. It was like a miniature cannon, but, unlike the first
small one, where the test had failed, this one would carry a special
projectile, that would be aimed at an armor plate set up on a big hill.

Tom's hope was that this big blast would show such pressure in
foot-tons, and give such muzzle velocity to the projectile, and at the
same time such penetrating power, that he would be justified in taking
it as the basis of his explosive, and using it in the big gun he
intended to make.

The preliminaries had been completed. The special steel tube had been
constructed, and mounted on a heavy carriage in a distant part of the
Swift grounds. A section of armor plate, a foot and a half in
thickness, had been set up at the proper distance. A new projectile,
with a hard, penetrating point, had been made--a sort of miniature of
the one Tom hoped to use in his giant cannon.

Now the young inventor and his friends were on their way to the scene
of the test, taking the powder and other necessaries, including the
primers, with them. Tom, Ned and Mr. Damon had some of the gauges to
register the energy expended by the improvised cannon. There were
charts to be filled in, and other details to be looked after.

"So General Waller won't be here?" remarked Ned, as they walked along,
Tom keeping a watchful eye on Koku.

"No," was the reply. "He has gone back to Sandy Hook. He wrote that his
health was better, and that he wanted to resume work on a new type of
gun."

"I guess he's afraid you'll beat him out, Tom," laughed Ned. "You take
my advice, and look out for General Waller."

"Nonsense! I say, Rad! Look out with those primers!"

"I'se lookin' out, Massa Tom. Golly, I don't laik dis yeah job at all!
I--I guess I'd better be gittin' at dat whitewashin', Massa Tom. Dat
back fence suah needs a coat mighty bad."

"Never you mind about the whitewashing, Rad. You just stick around here
for a while. I may need you to sit on the cannon to hold it dow