The engine is only one application of the principle of Tesla's turbine, because he has used the same idea on a pump and an air compressor as successfully as on his experimental engines. In his office in the Metropolitan Tower he has a number of models. Pointing to a little machine on a table, which consisted of half a dozen small disks three inches in diameter, he said: "This is only a toy, but it shows the principle of the invention just as well as the larger models at the power plant." Tesla turned on a small electric motor which was connected with a shaft on which the disks were mounted, and it began to hum at a high number of revolutions per second. "This is the principle of the pump," said Tesla. "Here the electric motor furnishes the power and we have these disks revolving in the air. You need no proof to tell you that the air is being agitated and propelled violently. If you will hold your hand down near the centre of these disks--you see the centres have been cut away--you will feel the suction as air is drawn in to be expelled from the outer edges. "Now, suppose these revolving disks were enclosed in an air-tight case, so constructed that the air could enter only at one point and be expelled only at another--what would we have?" "You'd have an air pump," was suggested. "Exactly--an air pump or a blower," said Doctor Tesla. "There is one now in operation delivering ten thousand cubic feet of air a minute." But this was not all, for Tesla showed his visitors a wonderful exhibition of the little device at work. "To make a pump out of this turbine," he explained; "we simply turn the disks by artificial means and introduce the fluid, air or water at the centre of the disks, and their rotation, with the properties of adhesion and viscosity immediately suck up the fluid and throw it off at the edges of the disks." The inventor led the way to another room, where he showed his visitors two small tanks, one above the other. The lower one was full of water but the upper one was empty. They were connected by a pipe which terminated over the empty tank. At the side of the lower tank was a very small aluminum drum in which, Tesla told his visitors, were disks of the kind that are used in his turbine. The shaft of a little one twelfth horsepower motor adjoining was connected with the rotor through the centre of the casing. "Inside of this aluminum case are several disks mounted on a shaft and immersed in water," said Doctor Tesla. "From this lower tank the water has free access to the case enclosing the disks. This pipe leads from the periphery of the case. I turn the current on, the motor turns the disks, and as I open this valve in the pipe the water flows." [Illustration: THE MARVELLOUS TESLA TURBINE The 200-horsepower engine, which a man could lift with one hand. How the Tesla Turbine compares in size with a man.] [Illustration: THOMAS A. EDISON AND HIS CONCRETE FURNITURE