Page:Popular Science Monthly Volume 45.djvu/633

Rh down, with, arrangements in the form of gates in the masonry wall separating it from the canal, by which water may be admitted to penstocks placed vertically in the slot and supplying the turbine wheels. A penstock, as many of our readers are aware, is a great tube, usually, in these days, of boiler plate, of a diameter running up, it may be, to thirteen feet, conveying water under head into the wheel case in which the turbine revolves.

In the present instance the penstocks, which are seven and a half feet in diameter, seem very small, considering that they each, supply a pair of wheels of five thousand horse power, but that is on account of the enormous pressure under which the wheels work, giving a greater power for a given volume of water than with the smaller heads more commonly used.

The turbines discharge their waste water into the tunnel above referred to, which is no less than six thousand seven hundred feet long, and which discharges into the chasm below the falls just past the Suspension Bridge.

The details of this tunnel, which was excavated through three shafts, one in the face of the cliff and two vertical ones, are as follows: Length, six thousand seven hundred feet, and sectional area three hundred and eighty-six square feet throughout, the average height and width being about twenty-one and nineteen feet respectively. The cross-section somewhat resembles a horseshoe. The excavation was much larger than the finished inside dimensions, on account of the subsequent lining with four courses of brick. The mouth of the tunnel has, besides, a lining on the top and sides of iron. The work has been done most substantially and is built to stay. The tunneling was done through strata of limestone and shale, and harder material was met with than had been expected in the beginning, so that the three million cubic feet of excavation has cut a very important figure in the total cost of the power plant. The tunnel has a grade of 0·7 per cent (seven feet fall per thousand length), and runs directly under the city of Niagara Falls to the lower river level.

The work of excavation was carried on on three benches, dividing the total height of twenty-six feet about into three equal portions.

The whole undertaking has been so entirely novel in many ways that the engineers in charge have had their resources taxed to the utmost in overcoming the various difficulties that presented themselves during the design and construction of the power house, electrical and hydraulic apparatus, and tunnel. The power-house building is as yet of comparatively small proportions, but is intended to be enlarged as the number of dynamos and turbines is increased. It might be thought, and was thought at first by some of the projectors of the scheme, that the great amount of power