# Page:Popular Science Monthly Volume 21.djvu/596

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THE POPULAR SCIENCE MONTHLY.

serve plant, as engines of a normal capacity of forty-two hundred can readily be forced to five thousand horse, or twenty per cent, to meet this extreme demand, and, with the generators arranged after Mr. Edison's plan, this per cent is an ample reserve. The maximum demand can, of course, be met either by forcing, or by running the entire plant at its normal rate, and forcing only in case of accident. To cover a district of three square miles, two distributing stations will be sufficient. The steam-dynamos may be taken as of two hundred horse each, working normally. The present steam-dynamos are of but one hundred and twenty-five horse, but they can be made two hundred horse with but slight increase of cost, which Mr. Edison contemplates doing in future installations. This will give thirteen steam-dynamos to one station and twelve to the other. These may each be placed at \$8,000, making a total for the two stations of \$200,000. That this is. a sufficient allowance will be evident upon considering the machines in detail. There are first the two hundred horse-power engines. No one will question that these can be obtained by a large buyer at \$18 per horse-power, or \$3,600 each.[1] This leaves \$4,400 to cover the cost of the dynamo. The material in these, as now being constructed, is as follows:

 Iron (wrought and cast) 40,700 pounds at 3 12 cents ${\displaystyle =}$ \$1,425 00 Zinc (cast) 680 "⁠ 6 "⁠ ${\displaystyle =}$ 40 80 Copper 3,440 "⁠ 28 "⁠ ${\displaystyle =}$ 963 20 ——— ————— 44,820 \$2,429 00

This leaves \$2,071 for the cost of construction, which will be recognized as more than enough, when it is remembered that the cost of the iron as above given includes its shaping, and that the copper on the armature is in the form of bars and disks, which, with suitable tools, can be expeditiously constructed.

Adding twenty-five per cent to the cost of material for the 200 horse machine, there is still left \$1,364 to be expended in construction. It seems to me, therefore, that \$8,000 is a safe estimate of the cost of such steam-dynamos. Regarding the boilers, the sectional or water tube boiler, on account of its freedom from dangerous explosions, the smaller space occupied by it, its higher efficiency, and less cost for repairs, is in every way the best suited for a purpose of this kind. Such a boiler set ready for use, including stack and apparatus for handling coal and firing, will cost \$20 per horse-power. The total boilers would therefore cost \$100,000, making the entire producing portion of the plant, exclusive of real estate, \$300,000.

As the Edison mains are now being laid, they will transmit a cur-

1. Mr. Edison informs me that engines of 200 indicated horse-power are being purchased by him for \$1,750 each, delivered in New York. This estimate is, therefore, much too high, but, as the comparison of plant in the text is based upon it, I have thought it best to let it stand, and point out the needed correction here.