"One for each oven," Mr. Nebett answered; "the keeper of the furnaces sets them when an oven is up to the required heat. Then, you see, it is easy to tell when they have been cooling long enough." "I should think," said Hamilton, "that making the barrel was the most important part of a gun, because, after all, that is the only part a bullet touches, and it must have to be exact. I've often thought of that, how the tiniest difference at the mouth of the barrel would at a thousand yards range cause it to be away off the mark." "It does have to be exact," his guide answered, "but that is a matter of care rather than of difficulty. In this next building we bore the rifle-barrels, just a simple boring process, as you see, but there are all sorts of precautions taken to insure absolute steadiness. As soon as a barrel is taken from the boring machine it is put through a test, to determine whether it is correct in size to the one-half of one-thousandth of an inch in diameter. If it is not as exact as that, it is set aside. That is only the first of a long series of tests, too. You would be surprised at the number of barrels that are rejected from the time of the first selection until the gun is completed. Here, for example, is perhaps the most sensational one." He led the boy to a small building, standing by itself in the middle of the yard, heavily built, and looking almost like a log cabin of the old type, made of great timbers. It was just a bit of a place, divided into two parts by a heavy timber wall. "What in the wide world is this for?" asked the boy. "I'll show you in a minute, I think we're just in time," the official said, as he led the way in. Hamilton followed him into the inner chamber. A long row of gun barrels was the first thing the boy noticed, the barrels all lying in slots. A gray-haired man was filling a heavy charge of powder behind each one. The guns were pointing into a bank of sand. "If you notice," said his guide, "you'll see that a little device, like the old percussion cap is right by each of those charges of powder. Are you all ready, Jim?" he queried, as the old man straightened up. "Yes, Mr. Nebett," was the reply. "All right," the other said, "we'll go into the room." He pointed out to Hamilton, as they passed from one part of this little building to the other, that each of these percussion caps was attached to a wire which ran through the wall to the little room into which they were going. "Look out, Mr. Nebett," said the old man, after he had closed and fastened the heavy door, "and you, young sir, don't be frightened," and he pulled the wire hanging overhead. There was a terrific explosion and a roar, and though Hamilton had been half expecting it, he jumped. Then he laughed. "I guess I did jump, after all," he said. "What was that for?"