the casing, and, being received by the blades, is whirled around and thrown outward into the surrounding chamber with a pressure proportionate to the square of the speed of the fan. The pressure thus produced is sufficient for blowing fires of all kinds, for removing small fragments of wood as well as finer particles from wood-cutting machines, for delivering illuminating gas to holders, and for other purposes requiring a pressure of not much over one pound per square inch. The centrifugal blower is the most efficient contrivance known

closed than when it is fully open. It is evident that when there is no discharge opening, the fan, together with the air between its blades, will spin around freely like any wheel. If the discharge gate be opened, air will flow about the fan blades to their outer ends, and so into the casing and thence to the pipe system; while air from the center of the fan follows out into the space between the blades, its speed increasing as it approaches the ends of the blades, whereupon it is hurled into the casing. It is the work done in accelerating the motion

of these particles of air, which explains the resistance to the movement of the fan; and this work is evidently in proportion to the quantity of air that passes along the fan blades, being zero when, by reason of the closing of either suction or discharge opening, no air is discharged, and being greatest when, with full opening, the volume of air passing is the maximum. The blades of the blower would fly in pieces before the speed could be increased sufficiently to produce an air pressure much over one pound per square inch.

An apparatus identical in principle is the centrifugal pump, which, handling far heavier fluids, produces by centrifugal action pressures measured by pounds. instead of ounces.

The Positive Blower

In direct competition with the centrifugal fan is the positive blower. Consider a fan having opposite blades, revolving in a closed casing, and carrying around with it the air confined between the blades. If a radial partition could be slipped in just after one of the blades had passed, the following blade would compress the air between it and the partition, and would force it out through a discharge pipe if one were provided near the supposed partition. This partition might be imagined to be momentarily withdrawn as each blade passed, so that successive portions of air would be trapped and compressed. The positive blower consists of a fan of this sort, with

leakage that would occur along the loose rolling contact of blades and abutment, and between blade and casing after a little wear had occurred. The mass of the moving parts, contrasted with the lightness of the working fluid, suggests the clumsiness of using a two-inch plank for a palm-leaf fan. The positive blower, limited to moderate speed by the noise and shocks of the heavy members of the apparatus, can produce pressures but little greater than those obtainable by means of the centrifugal blower.

The Air-Compressing Engine The air-compressor proper is a cylinder-and-piston machine like the common steam engine. It comprises two sets of valves, usually designed to be opened automatically by excess of pressure under them and to be closed by gravity or by the action of springs when the pressures become equal. The inlet valves open just after the piston commences its stroke, when the expansion of the compressed air remaining in the cylinder behind the piston has lowered the pressure

Perfectly correct action of automatic valves is not realized in practice. The valve must evidently be larger than the opening in its seat, so that the upper surface is larger than the area underneath reached by the lower pressure; consequently the valve will not open until the pressure below is greater than that above. To prevent destructive slamming of the valves, springs must be provided to force them to their seats just as the flow ceases and before the reverse stroke of the piston can cause much backward flow of air. The pressure of the springs acts to choke the flow through the valves, increasing the resistance they offer to the passage of air. Large compressors are therefore often provided with mechanically-actuated valves which are opened and closed smoothly at the proper moment by eccentrics and valve rods. Any of the steam-engine valve gears may be used for compressors, and designed by the same methods, observing only that the compressor is in every way a reversed steam engine, so that its discharge port and valves are duplicates of the

inlet details of the engine, while the engine exhaust and the compressor inlet valves are also similarly related.

Varying initial (or boiler) pressure is compensated, except in throttling engines, by varying time of inlet-valve closure or "cut-off;" and varying discharge pressure in a compressor calls for variation in time of opening of the discharge valves. In both cases, the means of variation constitute the chief problem for the designer.

Mechanically-moved inlet valves of compressors act always at the same points, opening a trifle after the piston starts on the intake stroke, and closing exactly at the end of the same stroke; but the discharge valves must open at the instant the piston has compressed the air in front of it to a pressure equaling that above the valves in the discharge

light springs, causing less resistance to the air passing through.

Where the expense of full mechanical action is warranted on account of the superior efficiency obtainable, poppet or rotary valves may be arranged to open by means of springs or air dashpots. The opening device is released either through the rising pressure in the cylinder easing the valve on its seat, and reducing the friction until the valve, when balanced, slips freely open; or through the same pressure acting on a piston attached to a pusher, the operation of which results. either in starting the valve in spite of friction or in lifting a catch and thereby freeing the spring or dashpot mechanism of the compressor.

pipe, and must close always at the same instant, at the end of the stroke. As the compressor may be working against a pressure greater or less than that regularly carried, the discharge valves must be so controlled as to open at whatever point is required by the pressure then being carried. The requirements are sometimes met by putting automatic liftvalves above, or even directly upon, the mechanical discharge valves, giving the combination the effect of valves automatic as regards their opening, but positively closed by the mechanism at the proper instant. Such lift-valves, being Such lift-valves, being shut off entirely at the proper closing instant, seat themselves without noise or shock, and may therefore have very

Compound Compressors

While low-compression pressures are accompanied by only moderate heating of the air during compression, the production of high pressures is attended with excessive heat and considerable increase in the volume of the compressed air. As the air leaving the cylinder soon resumes the normal temperature, and decreases in volume accordingly, the extra work done in compressing the increased volume is wasted. Compressing cylinders in operation are always cooled by water or otherwise; but it is impossible, even by spraying water into the cylinder, to keep the air from rising considerably in temperature. For high pressures, resort is therefore had to com

pound compression, the air being passed successively through larger low-pressure to smaller high-pressure cylinders, between which are located inter-coolers whose function is to restore the air to its original temperature before it enters on the next stage. The volume of the air is thus kept as small as possible; and the successive stages of compression result in producing the required pressure, with a minimum of loss from heating during the process. Two-stage machines are preferred to single-stage where air must be compressed to one-sixth or a smaller fraction of its volume at atmospheric pressure (measuring pressures from absolute vacuum); and three or more stages are required in compressing to one-sixth or less. Cylinder diameters are selected which will provide for about the same amount of work being done in each cylinder.

Most compressor problems deal with air taken directly from the atmosphere at its sea-level pressure; but, as at moderate elevation there is a marked decrease of the atmospheric pressure, compressors for high locations must deal with air at pressures below fifteen pounds absolute. Under such conditions the volume of air taken into the compressor at each stroke weighs less, and therefore less air is delivered by the compressor, while there is a corresponding decrease in the power to run the machine. The ratio of compression, and the rise in temperature, are proportionately increased, so that a two

stage compressor may be desirable for pressures that would call for only single cylinders at sea-level. Thus, at many of the mines in the Rocky Mountain region, the atmospheric pressure is as low as eleven pounds per square inch, so that 90 pounds' air pressure by gauge requires a compression ratio of nine to one, which is considerably beyond that proper for a single-stage compressor. In general, high-level compressors should be specially proportioned for their work. Methods of Driving

Like pumps and other machinery, compressors are direct-connected to engines or are driven through gears or belts from separate sources of power. The reciprocating piston compressor requires a varying effort to balance the cylinder pressure, since, during the stroke, the piston moves against an increasing air pressure, and finally against the full discharge pressure, in pushing out the contents of the cylinder. Direct-connected compressors are either "straight-line" (tandem), having steam and air pistons on the same piston rod, or they are connected to cranks set at an angle on a common shaft. The first method reduces floor space and cost, but requires very heavy fly wheels, and makes the machine liable to stop on a center if run much below full speed and capacity.

It is evident that steam used expansively supplies against the piston a force decreasing toward the end of the stroke,