In the same year Marble increased the safety of the bicycle by his invention of the rear-driven chain, and Schultz invented his chrome process of tanning leather. Both of these were important in their way; but in 1885 Cowles made a more important invention, that of reducing (and thereby producing) the metal aluminum from its oxide, called alumina, the chief constituent of clay. The usefulness of aluminum lies largely in its extreme lightness, and in the fact that when combined with certain metals, notably copper, it forms important alloys. During the same year, Welsbach invented his gas mantle, a valuable contribution to gas-lighting, and Bowers invented his hydraulic dredge, in which the act of dredging a channel or harbor was accomplished by hydraulic power. In the same year, Van Depoele invented a practical contact appliance for use in taking off the current from the overhead wires of electric railways. In 1886, Bell and Tainter invented the graphophone, an important improvement on the phonograph, and Elihu Thompson invented electric welding. This was an epochal invention, inaugurating as it did an entirely new art, and contributing enormously not only to the quickness of welding, but to its accuracy and strength. Many improvements have been made on this invention during the past few years, that have increased its scope and value. Many articles are now made in one piece that is really solid, though composed of several parts: for those parts are so firmly welded together that the joints cannot be seen and are as strong as any other parts. In the same year, Matteson invented his combined harvester and thresher. In the following year, Prescott invented his band wood saw, and McArthur and Forrest invented their process of extracting metals (especially gold and silver) from ores by the use of a solution of potassium cyanide, and greatly cheapened the work. In the same year, Tesla invented his system of multi-phase electric currents, which rendered possible the economical transmission of power over long distances, of which the first use was made in transmitting power derived from Niagara Falls. This was another invention of the first order of merit in brilliancy and originality of conception, excellence of constructiveness and usefulness of result. Its value has been only dimly appreciated by most men, because the invention does not stand continually before our eyes, like the telephone and electric light; for it cannot be seen at all. It is not a machine or instrument (in the common use of those words) but a system, actually invisible of itself, that governs the method of design, construction and operation of the visible dynamos, motors and conductors. Like the germ of life, we see not it, but only its manifestations.