Plymouth

3763

Pneumatic

a canopy, has been restored as nearly as pos- j
sible to its original position at the edge of the
water, where a simple but beautiful new can-
opy covers it.

Copyright, A. S. Burbank, Plymouth Rock.
Canopy Covering Plymouth Rock.

Plymouth Rock, a breed of domestic
fowls. See Poultry and Poultry Farming.

Plymouth Sound, an arm of the English
Channel, between Devonshire and Cornwall,
one of the famous roadsteads of the world.

P.M., post meridiem, 'after noon'; post
mortem, 'after death.'

Pneumatic Appliances, or mechanical
devices involving the use of air, range from
simple air-filled cushions to pumps, tools, and
engines worked by a steady supply of air. The '
earliest record of the use of compressed air
as a motive power dates from about the year
1700, when Denis Papin, in England, com-
pressed air by means of power derived from
a water wheel, and transmitted it through
tubes to a distance. In 1853 pneumatic power
was first utilized for commercial purposes in
London, England, when a tube 220 yards long
was constructed to carry telegraphic despatch-
es. The first successful application of com-
pressed air on a large scale was made in 1861
in connection with the construction of the
Mont Cenis Tunnel.

The U. S. Post Office Department in 1893
installed a system of pneumatic despatch at
Philadelphia. Pneumatic appliances may be
considered under seven heads, (i) Compressed
air has a wide application in a large variety of
apparatus wherein a simple mechanical push
or pull is required—ranging from a tiny cam-
era shutter, worked by a rubber bulb and hose,
to a large foundry hoist. (2) The foundations
for quay walls, dock entrances, and the piers
of bridges are often sunk to the required depth
by means of cylinders or caissons, from which
water is Deluded by forcing in air at a pres-

sure of from 10 to 30 Ibs. above that of the
outer atmosphere. This high pressure is con-
fined by a strong partition or diaphragm to
the working chamber, admission to which is
gained by means of an air lock. A similar sys-
tem is used in tunnelling with the hydraulic
shield through water-bearing strata. In these
cases the working length of tunnel is cut off by
a temporary wall, pierced for the air lock and
for the pipe supplying compressed air. The
air lock for tunnel shields is horizontal, for
foundation caissons vertical. This is a method
of construction used in the tunnelling under
the Hudson River for the Pennsylvania Rail-
road's tracks to New York City. (3) Attempts
to use compressed air as a propelling force in
military and naval practice have been con-
fined to guns of large calibre, and to the dis-
charge of projectiles containing high explo-
sives. The air was stored in tubes close to the
gun at a pressure of 1,000 Ibs. per square inch,
and was admitted to the breech of the gun
through a valve, so controlled that the range
of the projectile was governed by the amount
of air allowed to pass. No great success was
attained, however, and the guns were eventu-
al!}' discarded. (4) The conveyance of parcels
through the tubes.

(5) Transmission of power is effected by
compressing air at a central station, and sup-
plying it to consumers through a radiating se-
ries of main pipes, branching into others of
a less diameter. The chief system is in Paris,
and comprises 140 miles of mains, air being
supplied at a pressure of 75 Ibs. per square
inch, with a loss of 18 per cent, on the outside
of its zone. (6) For use as a motive power in
locomotives and automobiles, air is stored at
a high pressure Ci,ooo to 4,000 Ibs. per square
inch) in a steel reservoir carried on the car,
and is admitted thence at a working pressure
of 100 to 150 pounds into the driving cylinder.
While compressed-air locomotives and trucks
are still employed to some extent, their use is
now confined to special applications. Elec-
tricity has supplanted them in most cases. (7)
The force of suction obtained by exhausting
the air in a confined space is used in such ap-
pliances as grain elevators. In these the grain
is drawn up through a flexible pipe into the
receiver of the elevator, from which it either
descends through an air lock into barges be-
low, or is forced by air pressure through a pipe
leading to the top of the warehouse. Shavings
from wood-working mills are handled by a
similar system.

Domestic suction or vacuum cleaners have
come into such general use during recent years