One of the most interesting and valuable inventions of this or any century was that of the telephone, devised by Alexander Graham Bell, and first put to business use in 1877. For ten years Mr. Bell had been experimenting upon the possibility of conveying sound by means of the electric wire. In 1876 he had so far succeeded as to exhibit a pair of his instruments in successful operation at the Philadelphia Centennial Exhibition. In April of the next year the Cambridge Water Board, the Pennsylvania Railroad, and many other corporations ordered the instrument for practical service. From this time the business grew with incredible rapidity. The American Bell Telephone Company was incorporated, and in January, 1886, had in use 330,000 telephones. There were this year in various cities of the United States 752 telephone exchanges. The total length of telephone wires operated by this company was 114,371 miles. In addition to the system which centred in the Bell Company there were several competing telephone establishments in continual litigation with the Bell. The total mileage of telephone wires in the United States in 1887 was estimated at 130,000 miles. There were the same year 170,000 miles of telegraph wire, besides private lines. In 1893, the aggregate length of telegraph lines in the United States open to the public exceeded 210,000 miles. There were, besides, government and private lines to a length vast but not ascertainable. In addition to all this, the Bell Telephone Company, which conducted the main telephone business of the country, owned, the same year, 307,748 miles of wire, which the lines of other companies increased to 440,750 miles. [Illustration: Mass of wiring about 10 by 20 feet.] Under Side of a Modern Switchboard, showing 2,000 Telegraph Wires. From this account of our country's material advance after the war we purposely omit mention of the great economic progress at the South, as that has been already reviewed in Chapter IV. We must, instead, notice several events which decidedly checked the prosperity of these years. So soon as gold had ceased to circulate in 1862, speculation in it began, which was one of the most pernicious results of the depreciation of paper. The ups and downs in the gold price of the greenback from week to week and from day to day during the war were largely due to this cause. In 1869 a clique of speculators in New York thought to realize an immense fortune by cornering gold, a large proportion of the stock east of the Rocky Mountains being known to be in New York City.