But the Machine still continued growing. In 1900 Hewitt invented his beautiful mercury-vapor electric light, and in 1901 Santos-Dumont invented his air-ship and demonstrated its practicability by going around the Eiffel Tower in Paris in it and returning to the spot from which he started. This feat began that great succession of feats with dirigible balloons with which we are so familiar now, and which promise to be succeeded by a condition of world-wide transportation through the air. In 1900, the author of this book patented the method of controlling the movements of vessels, which consists in using radio telegraphy. This invention has recently been brought to the stage of practicality by the United States Navy. It was utilized in July, 1921, for steering the Iowa when bombed by airplanes. In 1903 came the first successful flight by aeroplane, which was made by the brothers Orville and Wilbur Wright at Kitty Hawk, North Carolina. This was an epochal adventure; it inaugurated an age which is already called the Aerial Age, and which will bring about changes so vast that our imagination cannot picture them. An interesting and instructive fact connected with this flight, and with the aeroplane in general, is that the aeroplane was not practicable and could not be made practicable before the internal-combustion engine had been invented and developed; because all preceding engines had been too heavy. This illustrates the fact occasionally adverted to in this book, that one of the most important factors in the influence of invention is that each new invention facilitates later inventions. _The influence of invention is cumulative._ In 1905, Elmer Sperry invented his gyroscopic compass which is unaffected by terrestrial magnetism and points to the true north. In 1907, he invented his gyroscopic stabilizer which reduces greatly the rolling of ships, aeroplanes, etc. Meanwhile, the endeavor to accomplish photography in color had been receiving persistent attention from many scientific experimenters, but without much practical success. The achievements of Becquerel, Lippman, Joly, Lumière, Finlay and others have doubtless laid the initial stepping stones; for color-photography by their efforts has been made an accomplished fact. As yet, however, the art is still in its infancy, and has not, therefore, reached the stage of maturity that enables us to estimate what importance it will eventually assume.