As we step from the door of the power-house and stand again beside the canal whose waters produce the wonderful effects we have witnessed in imagination, one question remains to be answered: What becomes of the water after it has passed through the turbine wheels down there in the depths? The answer is simple: All the water from the various turbines flows away into a great subterranean canal which passes down beneath the city of Niagara Falls, and discharges finally at the level of the rapids a few hundred yards below the Falls. The construction of this subterranean canal would in itself have been considered a great engineering feat a few decades ago; but of late years mountain tunnels, such subterranean railways as the London "tube system" and tunnels beneath rivers have robbed such structures of their mystery. It may be added that another such subterranean canal, to serve as a tail-race for one of the new Canadian plants, extends beneath the cataract itself, discharging not far from the centre of the Horseshoe Falls. Another of the power companies utilizes the water of the old surface canal which extends to the brink of the gorge some distance below the Falls. Yet another company on the Canadian side conveys water from far above the rapids in a gigantic closed tube to the brink of the gorge just below the Canadian Falls, above the point where their power-house is located. But the principle involved is everywhere the same. The idea is merely to utilize the weight of falling water. The water of Niagara River is of course no different from any other body of water of equal size. It is merely that its unique position gives the engineer an easy opportunity to utilize the potential energy that resides in any body of water--or, for that matter, in any other physical substance--lying at a high level. In due course, doubtless, other bodies of water, such as mountain lakes and mountain streams will be similarly put into electrical harness. The electrical feature is of course the one that most appeals to the imagination. But it may be well to recall that the ultimate source of all the power in question is gravitation. People fond of philosophical gymnastics may reflect with interest that, according to the newest theory, gravitation itself is, in the last analysis, an electrical phenomenon--a reflection which, it will be noted, leads the mind through a very curious cycle. THE EFFECT ON THE FALLS