There are eleven of these great turbine mechanisms, each with a supplying funnel of water and a revolving shaft extending upward to its companion dynamo, in the room in which we stand. Energy representing fifty-five thousand horse-power is incessantly transformed and made available for man's use in the subterranean building in which we stand. And there is not a pound of coal, not a lick of flame, not an atom of steam involved in the transformation. There are no dust-grimed laborers; there is no glare of furnace, no glow of heat, no stifling odor of burning fuel;--there is only the restful hum of the machinery that responds to the ceaseless flow of the silent and invisible waters. Day and night the mighty river here pulls away at its turbine harness; and man, having once adjusted that harness, may take his ease and enjoy the fruits of his ingenuity. As we return now to the top of the building, we shall view the spinning dynamos with renewed interest, and a few facts regarding their output of energy may well claim our attention. In their principle of action, as we have seen, all dynamos are alike,--depending upon the mutual relations between the wire-wound armature and a magnetic field. In the present case the magnets are made to revolve and the armatures are stationary, but this is a mere detail. There is one feature of these dynamos, however, which is of greater importance,--the fact namely that they operate without commutators, and therefore produce alternating currents. This fact has an important bearing upon the distribution of the current. Each of the dynamos before us generates the equivalent of five thousand horse-power of energy. There are eleven such dynamos here before us; there are ten more in the power-house on the other side of the canal, giving a total of one hundred and five thousand horse-power for this single plant; and there are five such plants now in existence or in course of construction to utilize the waters of Niagara, three being on the Canadian shore. When in full operation the aggregate output of these plants will be six or seven hundred thousand horse-power. SUBTERRANEAN TAIL-RACES