that amount. The reasons for this high frequency are mainly two: The first, that with any given alternating-current dynamo the number of alternations depends directly on the speed, and, as this must usually be high in order to get as much work as possible out of the machine, the periodicity is also high. The second reason is that in lighting work it is, of course, highly undesirable to employ a current of which the pulsations are so slow as to leave the incandescent filament or the arc visibly dimmer between separate beats, as we may call them, than during the passage of the full current strength. In the case in hand one is impressed with the effort that has been made to steer a middle course in the design of the generators so as to obtain a portion of
Fig. 7.—Elevation (part section) of Wheel Case, Pit, and Penstock.
the advantage of the direct current for motor work and of the alternating for transformation. The periodicity for the first portion at least of the electrical equipment is to be as low as twenty-five per second, and this at once limits the scope of the use of the current in the matter of electric lighting. Prof. Forbes states that lighting by the current direct is a comparatively small portion of the work in contemplation, and that the plant is rather to be regarded as essentially for power distribution. The expression, "lighting by the current direct," is used because a very important branch of the power work will be the lighting of the city of Buffalo. This is at present done by the ordinary direct-current arc machines operated by engines of some three thousand horse power. In changing over to the Niagara Falls power the whole
