The power of sending messages through space, in any direction, over great distances, is so enormous an addition to the utility of aircraft that a few words must here be said about wireless telegraphy. The discovery was made by the gradual researches of men of science. These researches had their beginning in a famous paper by James Clerk Maxwell, who subsequently became the first professor of experimental physics at Cambridge. His paper, _On a Dynamical Theory of the Electro-magnetic Field_, read to the Royal Society in 1864, contains a theoretical demonstration that electro-magnetic action travels through space in waves with the velocity of light. Twenty-three years later, in 1887, Heinrich Rudolf Hertz, of the University of Bonn, published the results of his experiments in producing these waves by means of oscillating currents of electricity. His investigations confirmed what Clerk Maxwell had proved mathematically. Thereafter progress was rapid, and during the closing years of the nineteenth century the problem of subduing the waves to the service of man was attacked and solved. In 1889 Professor Oliver Lodge was measuring electrical radiation. At Liverpool University College he constructed a Hertz radiator to emit the waves, and received them at various points of the building. Edouard Branly's invention of the 'coherer', an instrument designed to receive Hertzian waves, was communicated to the British Association at Edinburgh in 1893. During the same year Nikola Tesla published his researches on high frequency currents; on these much of the later work on wireless telegraphy was based. In 1895-6 William Rutherford set up at the Cavendish Laboratory apparatus by which he received signals in distant parts of Cambridge up to a distance of half a mile from the oscillator. Many other men of science, among whom was Captain H. B. Jackson, of the Royal Navy, were at work on the problem, when in 1896 Signor Guglielmo Marconi arrived in England with an apparatus of his own construction which ultimately brought wireless telegraphy to the stage of practical and commercial utility. By 1899 signals had been transmitted across the English Channel.