399. Backfall, 298. Balance-wheel, 419. Ball cock, 366, _367_. Balloon, fire, 323; gas, 347. Barometer, aneroid, 328, _329_; and weather, 331; Fortin's, _326_; meaning of, 237; principal, 238. Foci, conjugate, 262. Force, lines of, 116. Forces, component, 345. Free wheel, _440_. Furring-up of pipes, or stops. [Illustration: FIG. 49.] THE COMPENSATING GEAR. Every axle of a limited period, in some of.