EXAMPLE.-A round chimney 100 ft. high is to be used for a battery of boilers of 550 H. P. What should be the internal diameter?

SOLUTION.-Looking under column 100 in " "Height of Chimney in Feet" the nearest horsepower is 565, and the diameter corresponding is 60 in., which should be the interLal diameter of the chimney.

Chimneys are usually built of brick, though in some cases iron stacks are preferred. The external diameter of the base should be of the height, in order to provide stability. The taper of a chimney is from to in. to the foot on each side. The thickness of brickwork is usually 1 brick (8 or 9 in.) for 25 ft. from the top, increasing brick for each 25 ft. from the top downward. If the inside diameter is greater than 5 ft., the top length should be 14 bricks, and if under 3 ft., it may be

brick in thickness for the first 10 ft. A round chimney is better than a square one, and a straight flue better than a tapering one. If the due is tapering the area for calculation is measured at the top.

The flue through which the gases pass from the furnaces to the chimney should have an area equal to, or a little larger than, the area of the chimney. Abrupt turns in the flue or contractions of its area should be carefully avoided, as they greatly retard the flow of the gases. Where one chimney serves several boilers, the branch flue from each furnace to the main flue must be somewhat larger than its proportionate part of the area of the main flue.

SAFETY VALVES.

Balance the valve and lever over a sharp, knife-like edge, and measure the distance from the point of suspension to the fulcrum (center of pin on which the lever turns).

Let a =

b

=

distance thus measured in inches;

distance from center of valve to fulcrum in inches;

x = distance of weight from fulcrum in inches;

W =

Q

=

weight in pounds hung on lever;

weight of lever and valve in pounds;

A = area of safety valve in square inches;
pressure per square inch in the boiler.

p =

Exhaust steam from non-condensing engines usually con tains from 20% to 25% of water and oil, the latter being employed to lubricate the engine cylinders. Before exhaust steam is allowed to enter a heating system, the water and oil should be separated from it.

The effect of turning exhaust steam into a heating system is to form a back pressure on engine, which must be avoided as far as possible by using large steam-distributing pipes.

A direct connection to the steam boilers through a pressurereducing valve must be employed, to automatically furnish

steam to the heating system when the exhaust fails. A relief valve, also, should be placed upon the system, so that surplus exhaust steam may escape to the atmosphere.

To proportion an exhaust-heating system, it is necessary to know about how many square feet of radiating surface we should employ to properly condense the exhaust steam from the non-condensing engines. To do this we must first know the weight of steam that would be discharged from the engine.

From this must be deducted about 10% for condensation in the cylinders, etc., in order to obtain the real available weight of steam for heating purposes.

APPROXIMATE RATIO BETWEEN CUBIC CONTENTS AND RADIATOR SURFACE FOR EXHAUST HEATING.

The figures in the foregoing tables simply form a reasonable average, and allowance must be made for exposure, etc.

Each square foot of direct radiating surface gives off to the air around it about 14 thermal units per hour per degree of difference between the temperature of the steam and that of the surrounding air. This is equivalent to about lb. of steam per hr., or, in other words, about 4 to 44 sq. ft. of surface to each pound of steam to be condensed.

MACHINE DESIGN.

BLUEPRINTS.

Blueprint paper for copying tracings of plans and other drawings may be prepared as follows: Dissolve 1 oz., avoirdupois, of ammonia citrate of iron in 6 oz. of water, and in a separate bottle dissolve the same quantity of potassium ferricyanide in 6 oz. of water. Keep these solutions separate, and in a dark place, or in opaque bottles.

To prepare the paper, mix equal quantities of the two solutions, and with a sponge spread it evenly over the surface. Let the paper remain in a horizontal position until the chemical has set on the surface, which will take but a few minutes; then hang the paper up to dry. In preparing the paper darken the room by pulling down the shades, as direct rays of light affect sensitized surfaces. The prepared paper should be kept in a closed drawer, well covered with heavy paper, so that no light can come in contact with the sensitized surface; otherwise it will lose much of its value.

To make a blueprint from a tracing, lay the tracing with ink side down against the glass of the printing frame, then take the prepared paper, and place the sensitized surface down on the tracing. On the top of the paper place the felt cushion, on top of which place the hinged back of the printing frame, after which expose to the sunlight. The exposure will vary in sunlight from about 3 to 10 minutes. After the exposure, wash the paper thoroughly in a trough of cold water for about 10 minutes, and hang it up to dry.

The print after washing should be of a deep-blue color, with clear white lines. If the color is a pale blue, this indicates that the print has not had sufficient exposure, and if the lines of the drawing are not perfectly clear and white, that the exposure has been too long.

Corrections may be made on the print with an ordinary writing or ruling pen and a solution of washing soda, caustic potash, strong ammonia, or any other alkali. When any of these are mixed with carmine ink, the marks on the print will be red, thus making the corrections clear.

MACHINE TOOLS.

SPEED OF EMERY WHEELS.

The speed most strongly recommended by their manufacturers is a peripheral velocity of 5,500 ft. per min. for all sizes. All things being considered, it is stated that no advantage is gained by exceeding this speed. If run much slower than this, the wear on the wheels is much greater in proportion to the work accomplished, and if run much faster, the wheel is likely to burst.

SPEED OF GRINDSTONES.

Grindstones used for grinding machinists' tools are usually run so as to have a peripheral speed of about 900 ft. per min., and those used for grinding carpenters' tools at about 600 ft. per min. With regard to safety, it may be stated in general that with any size of grindstone having a compact and strong grain, a peripheral velocity of 2,800 ft. per min. should not be exceeded.

SPEED OF POLISHING WHEELS.

Polishing wheels are run at about the following peripheral speeds:

Leather-covered wooden wheels.

Walrus-hide wheels...

Rag wheels..

..7,000 ft. per min.

.8,000 ft. per min.

..7,000 ft. per min.

SPEED OF CUTS FOR MACHINE TOOLS. Brass: Use high speeds, about the same as for wood. Bronze: 6 to 18 ft. per min., according to alloy used. Cast or wrought iron: 20 ft. per min. is a good average for all machines, except millers. 30 is about the maximum. Machinery steel: 15 ft. on shapers, planers, and slotters. 20 to 45 on turret lathes, according to cut.

Tool steel: 8 to 10 ft.

Milling Cutters.-Gun metal, 80 ft. per min.; cast iron, 30; wrought iron, 35 to 40; machinery steel, 30. These are good speeds to adopt, with a view to economy, time required for regrinding, etc.