not the largest the government had. In
fact, Tom estimated the calibre to be less than twelve inches, but the
cannon was very long--much longer in proportion than guns of greater
muzzle diameter. Then, too, the breech, or rear part, was very thick
and heavy.

"He must be going to use a tremendous lot of powder," said Tom.

"He is," answered Captain Badger. "Some of us think he is going to use
too much, but he says it is impossible to burst his gun. He wants to
make a long-range record shot, and maybe he will."

"That's a new kind of breech block," commented Tom, as he watched the
mechanism being operated.

"Yes, that's General Waller's patent, too. They're going to fire soon."

I might explain, briefly, for the benefit of you boys who have never
seen a big, modern cannon, that it consists of a central core of cast
steel. This is rifled, just as a small rifle is bored, with twisted
grooves throughout its length. The grooves, or rifling, impart a
twisting motion to the projectiles, and keep them in a straighter line.

After the central core is made and rifled, thick jackets of steel are
"shrunk" on over the rear part of the gun. Sometimes several jackets
are put on, one over the other, to make the gun stronger.

If you have ever seen a blacksmith put a tire on a wheel you will
understand what I mean. The tire is heated, and this expands it, or
makes it larger. It is put on hot, and when it cools it shrinks,
getting smaller, and gripping the rim of the wheel in a strong embrace.
That is what the jackets of steel do to the big guns.

A big rifled cannon is loaded from the rear, or breech, just as is a
breech-loading shotgun or rifle. That is, the cannon is opened at the
back and the projectile is put in by means of a derrick, for often the
projectiles weigh a thousand pounds or more. Next comes the
powder--hundreds of pounds of it--and then it is necessary to close the
breech.

The breech block does this. That block is a ponderous piece of steel,
quite complicated, and it swings on a hinge fastened to one side of the
rear of the gun. Once it is swung back into place, it is made fast by
means of screw threads, wedges or in whatever way the inventor of the
gun deems best.

The breech block must be very strong, and held firmly in place, or the
terrific force of the powder would blow it out, wreck the gun and kill
those behind it. You see, the breech block really stands a great part
of the strain. The powder is bet