The lungs of the machine--its engines--are the crux of man's mechanical wisdom and skill.  Their marvelous reliability and intricacy are almost as awesome as the human anatomy.  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.