s if everyone was lifted from his feet. They
had all stood on their tiptoes, and opened their mouths to lessen the
shock, but even then it was terrific. The very ground shook--from the
roof of their cave small stones and gravel rattled down on their heads.
Their ear-drums were numbed from the shock. And the noise that filled
the valley seemed like a thousand thunderbolts merged into one.

Tom rushed from the bombproof, dropping the electric button. He caught
sight of his gun, resting undisturbed on the improvised carriage.

"Hurray!" he cried in delight. "She stood the charge all right. And
look! look!" he cried, as he pointed the glasses toward the distant
hillside. "There goes my projectile as straight as an arrow. There! By
Caesar, Ned! It landed within three feet of the target! Oh, you
beauty!" he yelled at his giant cannon. "You did all I hoped you would!
Thirty miles, Ned! Think of that! A two-ton projectile being shot
thirty miles!"

"It's great, Tom!" yelled his chum, clapping him on the back, and
capering about. "It's the longest shot on record."

"It certainly is," declared the foreman of the steel workers, who had
helped in casting many big guns. "No cannon ever made can equal it. You
win, Tom Swift!"

"Bless my armor plate!" gasped Mr. Damon. "What attacking ship against
the Panama Canal could float after a shot like that."

"Not one," declared Tom; "especially after I put a bursting charge into
the projectile. We'll try that next."

By means of compressed air the gases and some particles of the
unexploded powder were blown out of the big cannon. Then it was loaded
again, the projectile this time carrying a bursting charge of another
explosive that would be set off by concussion.

Once more they retired to the bombproof, and again the great gun was
fired. Once more the ground shook, and they were nearly deafened by the
shock.

Then, as they looked toward the distant hillside, they saw a shower of
earth and great rocks rise up. It was like a sand geyser. Then, when
this settled back again, there was left a gaping hole in the side of
the mountain.

"That does the business!" cried Tom. "My cannon is a success!"

The last shot did not go quite as far as the first, but it was because
a different kind of projectile was used. Tom was perfectly satisfied,
however. Several more trials were given the gun, and each one confirmed
the young inventor in his belief that he had made a wonderful weapon.

"If