580 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. Edi- 

 son'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 suffi- 

 cient. 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 contem- 

 plates doing in future installations. This will give thirteen steam-dy- 

 namos 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.* 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| cents = $1,425 00 



Zinc (cast) 680 "6 " = 40 80 



Copper 3,440 " 28 " = 963 20 



44,820 $2,429 00 



This leaves $2,071 for the cost of construction, which will be rec- 

 ognized 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 hand- 

 ling 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- 



* Mr. Edison informs me that engines of 200 indicated horse-power are being pur- 

 chased 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. 



