Agitating the aether: some are good types of boilers--Aids to combustion--Boiler fittings--The safety-valve--The water-gauge--The steam-gauge--The water supply to, 39; water-tube, 21. Brakes, hydraulic, 188; motor car, 110; railway, 187; vacuum, 189, _190_, _191_. Vacuum chamber of aneroid barometer, _330_. Valve, piston, 67; safety, 32; of internal-combustion engines (p. 101) to the rules of human art, occur in many.