Air-ships.--The aeroplane.--How the Wright aeroplane is kept afloat.--Submarines.--Some spinning tops that are useful.--The monorail-car.--Liquid air and the greatest cold.--The electric furnace and the greatest heat.--The wireless telegraph.--The wireless telephone.--Wonders of the alternating current.--X-rays and radium APPENDIX Brief notes on important inventions INDEX ILLUSTRATIONS FIG. MARCONI WIRELESS-TELEGRAPH STATION, CLIFDEN, IRELAND THE SAME STATION PHOTOGRAPHED BY DAYLIGHT 1--THE BATTLE OF SYRACUSE 2--GALILEO'S PENDULUM CLOCK 3--AN AIR THERMOMETER 4--TORRICELLI'S EXPERIMENT 5--GUERICKE'S AIR-PUMP 6--GUERICKE'S WATER BAROMETER 7--A LIFT-PUMP 8--A SIMPLE HYDRAULIC PRESS 9--HOW AN HYDRAULIC PRESS WORKS 10--AN HYDRAULIC PRESS WITH BELT-DRIVEN PUMP 11--NEWTON'S EXPERIMENT WITH THE PRISM 12--PAPIN'S ENGINE 13--THE NEWCOMEN ENGINE, IN REPAIRING WHICH WATT WAS LED TO HIS GREAT DISCOVERIES 14--CYLINDER OF WATT'S STEAM-ENGINE 15--A FLY-BALL GOVERNOR 16--A LEYDEN JAR 17--FRANKLIN'S KITE EXPERIMENT 18--VOLTA EXPLAINING HIS ELECTRIC BATTERY TO NAPOLEON BONAPARTE 19--THE FIRST ELECTRIC BATTERY 20--COUNT RUMFORD'S EXPERIMENT WITH THE CANNON, MAKING WATER BOIL WITHOUT FIRE 21--OERSTED'S EXPERIMENT 22--A COIL WITH A CURRENT FLOWING THROUGH IT ACTS LIKE A MAGNET 23--A BAR OF SOFT IRON WITH A CURRENT FLOWING AROUND IT BECOMES A MAGNET 24--TWO COILS WITH CURRENTS FLOWING IN THE SAME DIRECTION ATTRACT EACH OTHER 25--TWO COILS WITH CURRENTS FLOWING IN OPPOSITE DIRECTIONS REPEL EACH OTHER 26--ARAGO'S EXPERIMENT 27--ONE POLE OF A MAGNET SPINS ROUND A WIRE THROUGH WHICH AN ELECTRIC CURRENT FLOWS 28--WHEN A MAGNET IS THRUST INTO A COIL OF WIRE IT CAUSES A CURRENT TO FLOW IN THE COIL, BUT THE CURRENT FLOWS ONLY WHILE THE MAGNET IS MOVING 29--A COIL OF WIRE AROUND A COMPASS-NEEDLE 30--FARADAY'S INDUCTION-COIL 31--HISTORICAL APPARATUS OF FARADAY IN THE ROYAL INSTITUTION 32--FARADAY'S FIRST DYNAMO 33--FARADAY'S LABORATORY, WHERE THE FIRST DYNAMO WAS MADE 34--THE FIRST TRANSFORMER 35--THE "MAGNETIC FIELD" IS THE SPACE AROUND A MAGNET IN WHICH IT WILL ATTRACT IRON 36--MAGNETIC FIELD OF A HORSESHOE MAGNET 37--A DANIELL CELL 38--A GRAVITY CELL 39--SHOWING WHAT IS IN A DRY BATTERY 40--A STORAGE BATTERY, SHOWING THE "GRIDS" 41--A STORAGE-BATTERY PLATE MADE FROM A SHEET OF LEAD 42--STURGEON'S ELECTROMAGNET 43--AN ELECTROMAGNET WITH MANY TURNS OF INSULATED WIRE 44--AN ELECTROMAGNET LIFTING TWELVE TONS OF IRON 45--A DYNAMO WITH SIEMENS' ARMATURE 46--RING ARMATURE 47--FIRST DYNAMO PATENTED IN THE UNITED STATES 48--A DRUM ARMATURE, SHOWING HOW AN ARMATURE OF FOUR COILS IS WOUND