When both engines are going well, and synchronized to the same speed, the roar of the exhausts develops into one long-sustained and not inharmonious _boom-m-m-m-m!_ It is a song of pleasant melody to the pilot, whose ear is ever pricked to catch the first semblance of a "sharp" or "flat" note telling him that one or more of the twelve cylinders of each busy engine is missing fire and needs a little doctoring.
