Needle instruments--Influence of current on the magnetic needle--Method of reversing the current--Sounding instruments--Telegraphic relays--Recording telegraphs--High-speed telegraphy 127 Chapter VII.--WIRELESS TELEGRAPHY. The transmitting apparatus--The receiving apparatus--Syntonic transmission--The advance of wireless telegraphy 137 Chapter VIII.--THE TELEPHONE. The Bell telephone--The Edison transmitter--The granular carbon transmitter--General arrangement of a telephone circuit--Double-line circuits--Telephone exchanges--Submarine telephony 147 Chapter IX.--DYNAMOS AND ELECTRIC MOTORS. A simple dynamo--Continuous-current dynamos--Multipolar dynamos--Exciting the field magnets--Alternating current dynamos--The transmission of power--The electric motor--Electric lighting--The incandescent lamp--Arc lamps--"Series" and "parallel" arrangement of lamps--Current for electric lamps--Electroplating 159 Chapter X.--RAILWAY BRAKES. The Vacuum Automatic brake--The Westinghouse air-brake 187 Chapter XI.--RAILWAY SIGNALLING. The block system--Position of signals--Interlocking the signals--Locking gear--Points--Points and signals in combination--Working the block system--Series of signalling operations--Single line signals--The train staff--Train staff and ticket--Electric train staff system--Interlocking--Signalling operations--Power signalling--Pneumatic signalling--Automatic signalling 200 Chapter XII.--OPTICS. Lenses--The image cast by a convex lens--Focus--Relative position of object and lens--Correction of lenses for colour--Spherical aberration--Distortion of image--The human eye--The use of spectacles--The blind spot 230 Chapter XIII.--THE MICROSCOPE, THE TELESCOPE, AND THE MAGIC-LANTERN. The simple microscope--Use of the simple microscope in the telescope--The terrestrial telescope--The Galilean telescope--The prismatic telescope--The reflecting telescope--The parabolic mirror--The compound microscope--The magic-lantern--The bioscope--The plane mirror 253 Chapter XIV.--SOUND AND MUSICAL INSTRUMENTS. Nature of sound--The ear--Musical instruments--The vibration of strings--The sounding-board and the frame of a piano--The strings--The striking mechanism--The quality of a note 270 Chapter XV.--WIND INSTRUMENTS. Longitudinal vibration--Columns of air--Resonance of columns of air--Length and tone--The open pipe--The overtones of an open pipe--Where overtones are used--The arrangement of the pipes and pedals--Separate sound-boards--Varieties of stops--Tuning pipes and reeds--The bellows--Electric and pneumatic actions--The largest organ in the world--Human reeds 287 Chapter XVI.--TALKING-MACHINES. The phonograph--The recorder--The reproducer--The gramophone--The making of records--Cylinder records--Gramophone records 310 Chapter XVII.--WHY THE WIND BLOWS.