Why the wind blows--Land and sea breezes--Light air and moisture--The barometer--The column barometer--The wheel barometer--A very simple barometer--The aneroid barometer--Barometers and weather--The diving-bell--The diving-dress--Air-pumps--Pneumatic tyres--The air-gun--The self-closing door-stop--The action of wind on oblique surfaces--The balloon--The flying-machine 322 Chapter XVIII.--HYDRAULIC MACHINERY. The siphon--The bucket pump--The force-pump--The most marvellous pump--The blood channels--The course of the blood--The hydraulic press--Household water-supply fittings--The ball-cock--The water-meter--Water-supply systems--The household filter--Gas traps--Water engines--The cream separator--The "hydro" 350 Chapter XIX.--HEATING AND LIGHTING. The hot-water supply--The tank system--The cylinder system--How a lamp works--Gas and gasworks--Automatic stoking--A gas governor--The gas meter--Incandescent gas lighting 386 Chapter XX.--VARIOUS MECHANISMS. CLOCKS AND WATCHES:--A short history of timepieces--The construction of timepieces--The driving power--The escapement--Compensating pendulums--The spring balance--The cylinder escapement--The lever escapement--Compensated balance-wheels--Keyless winding mechanism for watches--The hour hand train. LOCKS:--The Chubb lock--The Yale lock. THE CYCLE:--The gearing of a cycle--The free wheel--The change-speed gear. AGRICULTURAL MACHINES:--The threshing-machine--Mowing-machines. SOME NATURAL PHENOMENA:--Why sun-heat varies in intensity--The tides--Why high tide varies daily 410 HOW IT WORKS. Chapter I. THE STEAM-ENGINE. What is steam?--The mechanical energy of steam--The boiler--The circulation of water in a boiler--The enclosed furnace--The multitubular boiler--Fire-tube boilers--Other types of boilers--Aids to combustion--Boiler fittings--The safety-valve--The water-gauge--The steam-gauge--The water supply to a boiler. WHAT IS STEAM? If ice be heated above 32 deg. Fahrenheit, its molecules lose their cohesion, and move freely round one another--the ice is turned into water. Heat water above 212 deg. Fahrenheit, and the molecules exhibit a violent mutual repulsion, and, like dormant bees revived by spring sunshine, separate and dart to and fro. If confined in an air-tight vessel, the molecules have their flights curtailed, and beat more and more violently against their prison walls, so that every square inch of the vessel is subjected to a rising pressure. We may compare the action of the steam molecules to that of bullets fired from a machine-gun at a plate mounted on a spring. The faster the bullets came, the greater would be the continuous compression of the spring. THE MECHANICAL ENERGY OF STEAM.