242 DISCOVERY CH. 



its practical applications, but the instances are innumerable of 

 investigations apparently quite useless in this narrow sense of 

 the word which have led to the most valuable results. Lord 

 Kelvin. 



There have been many improvements since the early 

 days of electric telegraphy, but they all depend upon 

 magnetic effects produced by electric currents. In 

 electricity, a far fleeter messenger has been found than 

 Puck, who boasted in Midsummer Night's Dream : 

 " I'll put a girdle about the earth in forty minutes." 

 The land wires and submarine cables now devoted to 

 telegraphic service would girdle the earth 250 times, 

 and with the telephone lines the total length is about 

 thirty-three million miles one-third of the distance 

 from the earth to the sun. 



Fourier's work which has been described as a 

 " mathematical poem " on the analytical theory of 

 heat, was published in 1822 long before electric cables 

 were dreamt ofbut it enabled Kelvin thirty years 

 later to attack a problem, the successful solution of 

 which created submarine telegraphy. 



Prof. Joseph Henry in the United States, Ampere in 

 France, and Faraday in England, gave particular 

 attention to Oersted's experiment and its consequences ; 

 and to their work we owe the construction of the electric 

 dynamo by which the current for lighting, traction, and 

 other purposes is now produced. They reasoned that 

 since electricity in motion could disturb a magnet, the 

 reverse should hold good, and a magnet in motion 

 should be able to produce an electric current in a con- 

 ductor near it. Both Henry and Faraday commenced 

 experimenting with the view of creating a current by 

 the action of a magnet, and both eventually succeeded. 

 Jn 1831 Faraday wrote to a friend : "I am busy just 



