Walter held the stick gingerly, as though he was afraid it might go off at any minute. "Don't be afraid of it," laughed McCarty. "It doesn't go off so very easy. It needs a sudden, hard jar, or a cap and fuse, to explode it. If I had swung that thirty-ton platform around on top of that stuff the machine would likely have been pretty badly smashed up, and maybe some of us killed." "Who could have put it there?" Walter asked. "Hard to say. Someone who wanted to put the machine out of business, of course. To be frank, however, it all points to Rooney, who had the shift before mine. He blew the whistle for me when his shift was up, and I left from the camp at once. We passed each other about halfway, so no one else would have had much chance to put anything in there, between his departure from the machine and my arrival. However, it has done no harm, so there's no use worrying about it now, but we had better look good, and see if there's any more of it scattered around." A close search, however, failed to reveal any more of the dangerous explosive, so the two boys crawled out from under the car and mounted the swinging steel platform, where McCarty showed the other which part of the cog-wheels they had seen below each lever controlled. There was a lever to move the car back and forth on its tracks like a steam engine, a lever to put on brakes, one to control the two-ton steel bucket, and another to raise and lower the long steel crane. "Best way for you to learn to run it is to come on as fireman," he advised. "It's a hot, dirty job, and long hours, but you've got to learn the steam part first before you can become a good runner. You've got to know enough about a steam engine to tell if your fireman is doing his part right--to know whether he is carrying too much or too little steam, and whether he keeps water enough in the boiler all the time. A careless fireman can easily blow up a boiler and wreck his engine, so it pays to keep an eye on your fireman." "All right. I'll come on as a fireman," agreed Walter, cheerfully.