Diving, _332_; electric, 119, _120_. Bellows of organ, 296; board of piano, 280. Spagnoletti disc instrument, 212. Sparking-plug, _102_. Spectacles, use of, 48. Focus, meaning of, 237; principal, 238. Foci, conjugate, 262. Force, lines of, 116. Forces, component, 345. Free wheel, _440_. Furring-up of pipes, 391. Fusee, drum and.