Pneumatic

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Pneumatic

that they must be classed aiming the most im-
portant pneumatic appliances. A great variety
of forms are seen, but all of them consist es-
sentially of a suction pump (generally motor
driven), a dust catcher, and a gathering nozzle
and pipe. A large variety of common air ap-
pliances may broadly be termed 'aspirators'
and 'atomizers,' these having fundamental
similarities. When a fluid (air, vapor, or li-
quid) is forced through a tube or across the
end of another tube, it draws along with it
particles of any fluid in the second tube, creat-

Pncumallc Paint Sprayer,
Slide feed for paint; flattened
conical nose piece to project
spray in thin sheet.

ing a suction therein, or aspirating the fluid
contained. If air thus entrains a liquid, a spray
emerges—that is, the device atomizes. The
common laboratory aspirator air pump does
not entrain air, but is a simple displacement
device. Atomizers range from small perfume
hand sprays to large painting machines, re-
quiring a considerable supply of compressed
air. Somewhat similar devices are used in sancl
blasting and in spraying cement on walls.
Compressed air blows the mixture much as it
does paint. In sand blasting the dry sand is
blown directly against the surface to be clean-
ed. Glass surfaces are frosted in this manner.
Pneumatic Despatch, the name given to
a method of sending parcels through a com-
paratively narrow tube by means of compress-
ed air at a spued of from 500 to T,OOO yards a
minute. In the "United States* where a closed-
circuit system is employed, great progress has
been made in the use of pneumatic power for
post office work. Cylinders of steel 7 by 24
inches, weighing about 13 lbsM and having a
capacity of about 800 cubic inches, are loaded
with letters (up to Coo), or whatever is to be
sent, and by an ingenious arrangement are in-
troduced into the tube without interrupting
the current. They are propelled at the rate of
the current, and on arrival at the terminus
are delivered into an air chamber which stops
them, also without interrupting the current.
In New York City, the three most important
post offices—Grand Central, Pennsylvania

Station, and the General Post Office—have a
set of tubes from each of the other two; and
from tht>e, branch tubes run to other central
poinis throughout Greater New York. The
tubes from the General Office to the Grand
Central Station (jl j miles), with three inter-
mediate stations, carry 6,000 Ibs. of letters
daily at the rate of 7 minutes each way.

Pneumatics is the study of the properties
of gases; but the term has fallen somewhat
out of use, and it is now more customary to
discuss the different aspects of the question un-
der the particular branches of physics con-
cerned. See GASES.

Pneumatic Tires, flexible tubes, inflated
with air, mounted on the rim of a wheel, were
first developed in connection with the bicycle,
and later modified and improved for motor car
application. The first bicycle tires were of
solid rubber. These were followed by 'cushion
tires,1 of nearly twice the diameter, and hav-
ing a small central air space. The next step
(J. B. Dunlop, 1888) was to make the diam-
eter larger and the walls thinner, and to pro-
vide a valve for inflating. Some of the tires
were 'single tube,' with the inner rubber skin,
strengthening fabric, and outer coat all vul-
canized together, Others were 'double tube/
with a thin inner envelope separate from a
heavier outer casing. The double-tube type
alone has survived in automobile service, on
account of its greater ease and permanence of
repair. In some bicycle tires the outer casing
had a small laced slit along its inner circum-
ference, through which the inner tube was in-
sert ed and removed. There were also several
'clincher* designs employing an endless inner
tube and an easily detachable casing. These
were the- prototypes of the modern automobile
tires, for which, sec MOTOR CAKR: Wheels and
Tires.

Pneumatic Tools, Prominent among
pneumatic appliances in general is a most use-
ful group of tools for working stone and metal
particularly. This group includes percussion,
coal punchers and cutters, riveting hammers,
chisels, etc. Percussion Drills consist essentially
of a cylinder and piston with the drill steel
held firmly in a chuck on the end of the piston
rod, together with a value gear for admitting
and exhausting compressed air (or steam) ai*
ternately from either side of the piston,
the whole being supported on a tripod or
bar with a hand-feed carriage. Com-
pressed air enters through the valve chest
on top, and is let out above or below the pis-
ton by a double-spool air-thrown valve, whose