They placed their blueprints and working notes in the safe, as usual, taking with them only those notes dealing with the object-compass and the X-plosive bullet, upon which they were still working. No one except Shiro knew that the original tracings, from which the blue-prints had been made, and their final, classified notes were always kept in the vault. They cautioned him and the three guards to keep a close watch until they returned. Then they set out in the biplane, to try out the new weapon in a lonely place where the exploding shells could do no damage. * * * * * They found that the X-plosive came fully up to expectations. The smallest charge they had prepared, fired by Crane at a great stump a full hundred yards away from the bare, flat-topped knoll that had afforded them a landing-place, tore it bodily from the ground and reduced it to splinters, while the force of the explosion made the two men stagger. "She sure is big medicine!" laughed Seaton. "Wonder what a real one will do?" and drawing his pistol, he inserted a cartridge carrying a much heavier charge. "Better be careful with the big ones," cautioned Crane. "What are you going to shoot at?" "That rock over there," pointing to a huge boulder half a mile away across the small valley. "Want to bet me a dinner I can't hit it?" "No. You forget that I saw you win the pistol trophy of the District." The pistol cracked, and when the bullet reached its destination the great stone was obliterated in a vast ball of flame. After a moment there was a deafening report--a crash as though the world were falling to pieces. Both men were hurled violently backward, stumbling and falling flat. Picking themselves up, they looked across the valley at the place where the boulder had stood, to see only an immense cloud of dust, which slowly blew away, revealing a huge hole in the ground. They were silent a moment, awed by the frightful power they had loosed. "Well, Mart," Seaton broke the silence, "I'll say those one-milligram loads are plenty big enough. If that'd been something coming after us--whether any possible other-world animal, a foreign battleship, or the mythical great sea-serpent himself, it'd be a good Indian now. Yes? No?" "Yes. When we use the heavier charges we must use long-range rifles. Have you had enough demonstration or do you want to shoot some more?" "I've had enough, thanks. That last rock I bounced off of was no pillow, I'll tell the world. Besides, it looks as though I'd busted a leg or two off of our noble steed with my shot, and we may have to walk back home." An examination of the plane, which had been moved many feet and almost overturned by the force of the explosion, revealed no damage that they could not repair on the spot, and dusk saw them speeding through the air toward the distant city. In response to a summons from his chief, Perkins silently appeared in Brookings' office, without his usual complacent smile.