The intensity of draught required varies with the kind and condition of the fuel, and the thickness of the fires. Wood requires the least, and fine coal or slack the most. To burn anthracite slack to advantage, a draught of 14 inch of water is necessary, which can be attained by a well-proportioned chimney 175 ft. high.

A round chimney is better than square, and a straight flue better than tapering, though it may be either larger or smaller at top without detri

ment.

The effective area of a chimney for a given power, varies inversely as the square root of the height. The actual area, in practice, should be greater, because of retardation of velocity due to friction against the walls. On the basis that this is equal to a layer of air two inches thick over the whole interior surface, and that a commercial horse-power requires the consumption of an average of 5 pounds of coal per hour, we have the following formula:

In which H-horse-power; h-height of chimney in feet; E-effective area, and A-actual area in square feet; S-side of square chimney, and D=dia. of round chimney in inches. The following table is calculated by means of these formulæ, by Wm. Kent:

SIZES OF CHIMNEYS WITH APPROPRIATE HORSE-POWER OF BOILERS.

Height of Chimneys, and Commercial Horse-Power.

60 ft. 70 ft. So ft. 90 ft. 100 ft. J10 ft. 125 ft. 150 ft. 175 ft. 200 ft.

The external diameter at the base should be one-tenth the height, unless it be supported by some other structure. The "batter" or taper of a chimney should be from to 1/4 inch to the foot on each side.

Thickness of brick work: one brick (8 or 9 inches) for 25ft. from the top, increasing 1⁄2 brick (4 or 41⁄2 inches) for each 25ft. from the top downwards.

If the inside diameter exceed 5 ft. the top length should be 11⁄2 bricks, and if under 3 ft. it may be 1⁄2 brick for ten feet.

STEAM.

A cubic inch of water evaporated under ordinary atmospheric pressure is converted into I cubic foot of steam (approximately).

Steam at atmospheric pressure flows into a vacuum at the rate of about 1550 feet per second, and into the atmosphere at the rate of 650 feet per second.

The specific gravity of steam (at atmospheric pressure) is .411 that of air at 34° Fahrenheit, and .0006 that of water at same temperature.

27,222 cubic feet of steam weigh one pound: 13,817 cubic feet of air weigh 1 pound.

Locomotives average a consumption of 3000 gallons of water per 100 miles run.

The best designed boilers, well set, with good draught and skillful firing, will evaporate from 7

to 10 lbs. of water per pound of first-class coal. The average result is from 25 to 60 per cent below this.

In calculating horse-power of Tubular or Flue boilers, consider 15 square feet of heating surface equivalent to one nominal horse-power.

One square foot of grate will consume on an average 12 lbs. of coal per hour.

Steam engines, in economy, vary from 20 to 60 lbs. of feed water and from 2 to 7 lbs. of coal per hour per indicated H. P.

Condensing engines require from 20 to 30 gallons of water to condense the steam represented by every gallon of water evaporated-approximately for most engines, we say, from 1 to 1% gallons per minute per I. H. P. Jet condensers do not require quite as much water for condensing as Surface condensers.

Surface condensers should have about 2 square feet of tube (cooling) surface per horse-power of steam engine. It is absolutely necessary to place air-pumps below condensers to get satisfactory results.

WEIGHT AND COMPARATIVE FUEL VALUE OF WOOD.

I Cord Air-dried Hickory or Hard Maple weighs about 4500 lbs., and is equal to about 2000 lbs. coal.

I Cord Air-dried White Oak weighs about 3850 lbs., and is equal to about 1715 lbs. coal.

I Cord Air-dried Beach, Red Oak or Black Oak, weighs about 3250 lbs., and is equal to about 1450 lbs. coal.

1 Cord Air-dried Poplar (whitewood), Chestnut or Elm, weighs about 2350 lbs., and is equal to about 1050 lbs. coal. I Cord Air-dried Average Pine, weighs about 2000 lbs., and is equal to about 925 lbs. coal.

*

From the above it is safe to assume that 24 lbs. of dry wood is equal to 1 lb. average quality of soft coal, and that the full value of the same. weight of different woods is very nearly the same—that is, a pound of hickory is worth no more for fuel than a pound of pine, assuming both to be dry. It is important that the wood be dry, as each 10 per cent of water or moisture in wood will detract about 12 per cent from its value as fuel.

*Usually considered the standard point of efficiency-Condenser and Air Pump being well proportioned.