APPROXIMATE RULES AS TO BOILERS (APPLEBY).

Boilers. An ordinary furnace requires 24 lbs. or 300 cubic feet of air for the consumption of each pound of coal, but by means of the fan or steam jet the quantity may be reduced to 18 lbs. or 220 cubic feet. From 13 to 20 lbs. of fuel may be consumed per square foot of firegrate, and in Cornish, Lancashire, or egg-end boilers about three-fourths of a square foot are required to evaporate a cubic foot of water.

For each nominal horse-power a boiler of any of the types enumerated above requires approximately :

1 cubic foot of water per hour.

1 cubic yard capacity.

1 square foot of fire-grate surface.

1 square yard of heating surface.

Formula for finding the Heating and Grate Surfaces.

F= Fire-grate surface in square feet.
NNominal horse-power.

H= Heating surface in square feet.

Fire-bars should incline from the fire-door downwards at least 1 in 12, and cylindrical boilers should be set with an inclination of onehalf inch in 10 feet towards the blow-off cock.

The dead-plate of a furnace should be 2 feet 8 inches above the floor line of stoke-hole.

Water Level in Boilers.—The least depth of water above the furnace flue should be 4 inches, and the working depth about 9 inches.

Priming.-A large dome or steam chest tends to prevent priming, but a perforated tube in which the steam is collected inside the boiler is a still greater aid in this direction.

The following formula for finding the areas of chimneys for stationary boilers may be useful:

Q= lbs. of coal consumed per hour.

H = Height of chimney in feet.

PIndicated (not nominal) horse-power of engine.

A The area of the top of the chimney in square inches.

The diameter of the base of a chimney should not be less than onetenth of the height.

Multitubular boilers (not marine) each nominal horse-power requires approximately—

1 cubic foot of water per hour.
square foot of fire-grate surface.

10 square inches sectional area of tubes.

13 square inches of flue area.

6 square inches of chimney area.

2 cubic feet of steam space.

8 cubic feet total capacity.

10 square feet of heating surface, if the whole tube surface be taken as effective.

NOTES ON WATER-WHEELS AND TURBINES (APPLEBY).

The maximum theoretical efficiency for undershot water-wheels is obtained when the speed of the periphery is half the speed of the driving stream; but experience has shown that the most suitable speed for paddle-wheels in an open current is 40 per cent. of the speed of the stream. The efficiency of undershot wheels is seldom more than 27 to 30 per cent. of the power expended, but in Poncelot's undershot wheel it reaches 50, 55, and even 60 per cent.

Breast-wheels give an efficiency of about 75 per cent. of power expended with a velocity of 5 feet per second at the periphery, but in an exceptional case as much as 93 per cent. was registered by M. Morin. The usual velocity for breast-wheels is about 6 feet per second at the periphery; maximum 3 feet 6 inches, maximum 7 feet. Overshot wheels answer well for falls of from 13 to 20 feet, and give an efficiency of 70 to 75 per cent. The power of any of the above forms of wheels may be obtained by multiplying the weight of the water used by the height it falls through and the factor of efficiency given.

In Fourneyron's turbines the factor of efficiency varies from ·79 to 24 according to the rate of regulating sluices, from which it is evident that they are far more economical when working at full power.

For outward flow turbines it has been found by experience that the height of the orifices at the circumference of the wheel should be onetenth the diameter of the wheel; that the sum of the shortest distances between the buckets should equal the diameter of the wheel; that the width of the crowns should be four times the shortest distance between the buckets; and that the sum of the shortest distances between the curved guides taken near the wheel should be equal to the interior

diameter of the wheel. For any fall greater than 40 feet the first rule should be modified, the height of orifices decreasing as the fall increases. By attention to these points an efficiency of 75 per cent. may be realised. In turbines with a downward flow, such as Fontaine's, an efficiency of 70 per cent. may be reached when working at full power. The best speed for the periphery of these wheels is 55 per cent. of the theoretical velocity due to the head.

Jonval's turbines give an efficiency of 72 per cent. under full work, and the best velocity for the exterior of the wheel is 70 per cent. of that due to the head. The inward flow, or Thompson's vortex turbine, has realised an efficiency of 77 per cent. or even more; the speed of periphery in this class of turbine should be half of that due to the head. It is stated that the Girard turbine at the Paris Waterwork has reached an efficiency of 87 per cent.

TABLES.

Whitworth's Threads is the designation given to a standard set of threads introduced by Sir Joseph Whitworth, and now universally used. The following gives the diameter and number of threads per inch:

The angle of the threads is 55°, with one-third of the depth rounded off top and bottom.

General Rules in Screw-cutting.—In cutting all threads, it is better to have a nut or gauge to work to, especially if you have many of one size to cut. The angle of the tool for Whitworth's standard V threads must be 55°. Finish with light cuts, using soap-and-water as a lubricant, except for cutting steel, when oil answers better. See that your carrier or driver has a proper hold before cutting so as not to slip, or the result is a stripped thread. Depth of the thread must be the pitch of the screw, but th of depth must be rounded off at top and bottom. The number of threads per inch in square threads, half those of V threads.

SCALE FOR GAS-PIPE THREADS PER INCH.

TABLE OF WEIGHTS AND MEASURES IN ENGLISH WITH THEIR
EQUIVALENTS IN FRENCH.