"Hold it!" Colonel Welsh cried, and laughed shortly. "You're flying one wing low this time. I said in the wire that I was coming out, but I didn't say _how_, or _what_ time I'd arrive. Afraid you're off on the wrong scent there, Dawson." "Maybe, maybe not," Dave said doggedly. "But that chap didn't have a crack at us tonight just for gunnery practice. He was shooting for keeps. He knew darn well who was in this plane--and he was out to get us. He--" Dave didn't finish. At that moment the right outboard engine of the Lockheed lost revs fast and began to sputter and clatter. Dave snapped his eyes at the dash dials, and sucked in his breath sharply as he saw the oil pressure needle sliding around the face of the dial toward the zero peg. However, even as he glanced at the needle, it stopped swinging back and promptly climbed upward again. The engine stopped sputtering and clattering, and once again sang its full throated song of power. The tiny lump of ice remained in Dave's chest, however. He glanced sidewise at Freddy Farmer and saw the corners of the English youth's mouth tighten a bit. "What the devil was that?" Colonel Welsh demanded in a sharp tone. "Something wrong with the engine?" "Not now," Dave said with an easiness he didn't feel. "Guess it picked up a bit of ice but got rid of it in time. Anyway, she's back where she should be. As I was saying, that lad tonight was out for blood. So it must follow that somebody knew where you were going, when, and how. Don't you think so, sir?" Dave spoke the words, but it was really just an effort to keep the conversation going. The lump of ice in his chest was hurting him again, and he was feeling far from calm and collected. The way the oil pressure of the right outboard engine had dropped told him that there was trouble ahead. Many people claim that the carburetor is the heart of an engine, and probably it is, if you want to look at it that way. However, countless hours in the air had proved to Dave that your real danger signal is when oil pressure starts dancing around. Engines can run, often for a long, long time, when the carburetor is out of whack and the engine is getting a bad feed. But let oil pressure go screwy and you'll have real trouble on your hands. There are no halfway measures about oil. It has to be right or your engine is worth no more than its weight in junk. Gasoline is food for an engine, but oil is its life blood. If it hasn't got the proper amount it dies, but definitely!