THE PNEUMATIC TRANSIT COMPANY AND THE FIRST PNEUMATIC TUBES FOR THE
TRANSPORTATION OF UNITED STATES MAIL.
=Organization.=—Early in the year 1892 several Philadelphia gentlemen
organized a corporation and obtained a charter in the State of
New Jersey to construct, lay, and operate pneumatic tubes for the
transmission of United States mail, packages, merchandise, messages,
etc., within the States of New Jersey and Pennsylvania. The corporation
was styled the Pneumatic Transit Company. Mr. William J. Kelly was
elected president, and the company is still under his management.
[Illustration: WM. J. KELLY,
President of the Pneumatic Transit Co.]
=Aim and Object of the Company.=—When the Pneumatic Transit Company
was formed, it was the aim and object of its promoters to construct
an extensive system of underground tubes in the City of Philadelphia
which would serve, first, for the rapid transmission of mail, second,
for the quick delivery of merchandise from the large retail stores,
third, for the transmission of telegrams or messages within the city
limits, and, fourth, to conduct a general local express business with
greater speed than can be done in any other manner. To accomplish
this result sub-stations were to be located six or eight blocks apart
throughout a large portion of the city, and a central station was to be
established in the centre of the business section. Stations were
also to be established in the more important retail stores and large
office buildings, and all of the stations were to be connected by tubes
forming one large system.
For the transmission of mail it was planned to connect the main
post-office with the sub-post-offices by tubes of a size large enough
to carry all of the first-class and most of the other classes of
mail matter. The sub-post-offices would be divided into groups, all
of the offices in one group being connected to the same line, which
would terminate at the main post-office. Most of the business would
be between the main and individual sub-offices; in addition to this
there would be some local mail sent between the sub-offices which, for
offices in the same group, could be despatched directly without passing
through the main office. The advantages to be gained by the use of
these tubes over the present wagon service are very apparent. It places
all the sub-post-offices in almost instant communication with the main
office and with each other.
It was a part of the general plan to lay tubes from the main
post-office to the railway stations, thereby hastening the despatch and
receipt of mails to and from the trains.
It was expected that the bulk of the business would consist in the
delivery of parcels from the retail stores to the private houses in the
residence sections of the city. Of course it would not be practicable
to lay a tube to each house, but with a station not more than four or
five blocks away, the parcels would be sent through the tube to the
nearest station, and then delivered by messengers to the houses with a
minimum loss of time. Ladies could do their shopping and find their
purchases at home when they returned.
The same tubes used for parcel delivery would also be used for a
district messenger service. With numerous public stations in convenient
locations, all the advantages of the European system would be realized
in the quick despatch of letters and telegrams. Every one knows how
much time is consumed by district messenger-boys in the delivery of
messages, especially when they have to go long distances, and no
argument is required to show that this time would be very much reduced
by the use of pneumatic tubes, besides prompt delivery would be made
much more certain.
The tubes of this system were to be six or eight inches in diameter,
with a few small tubes in localities where the message service is very
heavy.
Without going more into detail, such were in brief the plans of the
promoters of this new company; but before launching such an enterprise,
involving a large amount of capital, there were many engineering and
mechanical problems to be solved. It was not simply a question of
obtaining tubes and laying them in the streets, but ways and means
for operating them must be devised. Up to this time only small tubes
had been used for the transmission of telegrams, messages, cash, and
other light objects. Now it was proposed to transmit heavy and bulky
material. There was no experience for a guide.
=The Clay-Lieb Patents.=—The Pneumatic Transit Company at this time
turned to the Electro-Pneumatic Transit Company, of New Jersey, a
national company that had been in existence since 1886, and which
claimed to own valuable patents, for the ways and means to carry out
its new enterprise. The patents were those of Henry Clay and Charles
A. Lieb, and the rights to use them in the State of Pennsylvania
were procured by the Pneumatic Transit Company, under a contract
entered into between the two companies. The patents claimed to cover
a practical working system by which a large number of stations
could be connected to a system of main and branch tubes, with
electrically-operated switches at the junctions of the branches with
the main lines. Any person who gives the subject a little thought
will at once see the advantages of such a system if it could be made
to operate. Up to the present time only single- or double-line tubes
have been used, without branches. In the European systems, frequently
several stations are located along a line, but the carriers must stop
at each station, be examined, and if they are destined for another
station, they must be redespatched. The cash systems used in many
of our large stores have independent tubes running from the central
cashier’s desk to each station about the store. It is plain to be seen
that, if several of these stations could be connected by branches to
a main tube, a large amount of tubing would be saved—a most desirable
result. The advantages of such a system would be still greater for long
lines of tube laid under the pavements, extending to stations located
in different parts of a large city. It was such a result that the
patents of Clay and Lieb aimed to accomplish.
In order to demonstrate the practicability of the system, the
Electro-Pneumatic Transit Company had constructed in the basement of
the Mills Building, on Broad Street, New York, a short line of small
brass tubing, about two or three inches in diameter, with one branch,
thus connecting three stations together. The tube was very short,
probably not more than two hundred feet in length. The air-pressure
required was very slight, probably not more than an ounce or two, being
supplied by a small blower run by an electric motor.
At the junction of the branch and the main tube was located a switch
that could be moved across the main tube and so deflect the approaching
carrier into the branch. This switch was moved by an electro-magnet,
or solenoid, that could be excited by pressing a button at the station
from which the carrier was sent. When the carrier passed into the
branch tube it set the switch back into its normal position, so that a
second carrier, following the first, would pass along the main tube,
unless the switch was again moved by pressing the button at the sending
station.
This tube in the Mills Building worked well, but it was of a size
only suited to the transmission of cash in a store or other similar
service. It could not be said, because this tube worked well, that
a larger and longer tube with numerous branches would work equally
well. In fact, there are several reasons why such a tube would not
operate satisfactorily. The method of operating the switches was
impracticable. Suppose the branch tube had been located two miles away
from the sending station and that it would take a carrier four minutes
to travel from the sending station to the junction of the branch tube.
Again, suppose that we have just despatched a carrier destined for a
station on the main line beyond the junction, and that we wish to
despatch the second carrier to be switched off into the branch tube,
we must wait at least four minutes, until the first carrier has passed
the junction, before we can press the button and set the switch for
the second carrier which may be on its way. How are we to know when
the first carrier has passed the junction, and when the second will
arrive there, in order that we may throw the switch at the proper time?
Must we hold our watch and time each carrier? It is plain that this is
not practical. I take this as an illustration of the impracticability
of the Clay-Lieb System as constructed in the Mills Building when
extended to practical dimensions. I will not describe the mechanism and
details of the system, which are ingenious, but will say in passing
that the automatic sluice-gates, which work very well in a three-inch
tube with carriers weighing an ounce or two and air-pressures of only
a few ounces per square inch, would be useless and could not be made
to operate in a six-inch tube with carriers weighing from eight to
twenty-five pounds and an air-pressure of from five to twenty-five
pounds per square inch. For further information the reader is referred
to the patents of Clay and Lieb.
=Franchises and First Government Contract.=—In the spring of 1892
an ordinance was passed by Common and Select Councils, and signed
by the Mayor of the City of Philadelphia, permitting the Pneumatic
Transit Company to lay pneumatic tubes in the streets of that city.
At the time this franchise was granted negotiations were in progress
with the post-office department, in Washington, for the construction
of a six-inch pneumatic tube, connecting the East Chestnut Street
sub-post-office, at Third and Chestnut Streets, with the main
post-office, at Ninth and Chestnut Streets, for the transmission of
mail. This sub-post-office was selected because more mail passes
through it daily than any other sub-office in the city, it being
located near the centre of the banking district. Negotiations were
delayed by various causes, so that the contract with the government was
not signed until October, 1892.
[Illustration: FIG. 4.
[Illustration: FIG. 5.
SIX-INCH PNEUMATIC TUBES IN PROCESS OF BORING AT THE SHOP OF A.
FALKENAU, PHILADELPHIA, PA.]
=Search for Tubes.=—It was at this time that the writer was first
employed by the Pneumatic Transit Company, as engineer, to superintend
the construction of this line. The company commenced at once to
carry out its contract with the United States government, both the
post-office department and the company being very desirous of having
the work completed before winter. The time was very short for such
an undertaking, but wrought-iron tubes had already been ordered of a
well-known firm who manufacture pipe and tubing of all kinds. After
waiting four or five weeks the first lot of tubes were finished,
but upon inspection it was found that they were not sufficiently
accurate and smooth on the interior to permit of their being used
for the purpose intended. The next thing that suggested itself was
seamless drawn brass tubes. While they would be very expensive, the
process of manufacture makes them eminently suited for the purpose,
but they could not be obtained in time. A city ordinance prohibits
the opening of the streets of Philadelphia during the winter months
except in extreme cases. Accurate tubes must be had, and had quickly.
It then occurred to the writer that it might be possible to bore a
sufficient quantity of ordinary cast-iron water-pipe and fit
the ends accurately together to answer our purpose. Inquiry was made
at nearly all the machine-shops in the city, to ascertain how many
boring-machines could be put upon this work of boring nearly six
thousand feet of six-inch pipe. It was found impossible to get the work
done in time, if it was to be done in the usual manner of boring with
a rigid bar. At last a man was found in Mr. A. Falkenau, engineer and
machinist, who was prepared to contract for the construction of twelve
special boring-machines and to bore all the tubing required. Suffice it
to say, that the machines were built, and about six thousand feet of
tubes were bored, between November 8 and December 31.
[Illustration: FIG. 6.
_PIPE BORING APPARATUS._]
=Method of Manufacturing Tubes.=—The process of boring was novel in
some respects, and might be termed reaming rather than boring. Figs.
4 and 5 show the interior of the shop and the twelve machines. Fig. 6
is a drawing of one of the machines. A long flexible bar rotated the
cutter-head, which was pulled through the tube, in distinction from
being pushed. In order to give the feeding motion, a screw was attached
to the cutter-head and extended through the tube in advance of it. The
feed-screw was drawn forward by a nut attached to a hand-wheel located
at the opposite end of the tube from which the boring began. Since it
was not necessary that the tubes should be perfectly straight, a method
of this kind was permissible, in which the cutters could be allowed to
follow the cored axis of the tube. Air from a Sturtevant blower was
forced through the tubes during the process of boring, for the double
purpose of clearing the chips from the cutters and keeping them cool.
After the tubes were reamed, each piece had to be placed in a lathe,
have a counter-bore turned in the bottom of the bell, and have the
other end squared off and turned for a short distance on the outside to
fit the counter-bore of the next section.
=Laying and Opening the Tubes for Traffic.=—The first tubes were laid
about the middle of November, but December 1 came before the work was
completed and special permission had to be obtained from the city
to carry on the work after that date. All work was suspended during
the holidays in order not to interfere with the holiday trade of the
stores on Chestnut Street. Severe frosts prevailed at that season, so
that when the work was begun again, after the holidays, bonfires had
to be built in the streets to thaw out the ground in order to take up
the paving-stones and dig the trench for the tubes. Several times the
trench was filled with snow by unusually heavy storms. Notwithstanding
all these delays and annoyances, the work was pushed forward, when
a less determined company would have given it up, and was finally
completed. A formal opening took place on February 17, 1893, when Hon.
John Wanamaker, then Postmaster-General, sent through the tube the
first carrier, containing a Bible wrapped in the American flag.
It was certainly a credit to the Pneumatic Transit Company and its
managers that they were able to complete this first line of tubes so
quickly and successfully under such trying circumstances. The tubes
have been in successful operation from the opening until the present
time, a period of nearly four years, and the repairs that have been
made during that time have not required its stoppage for more than a
few hours.
In the summer of 1895 the sub-post-office was removed from Chestnut
Street to the basement of the Bourse (see Fig. 7). This required the
abandonment of a few feet of tube on Chestnut Street and the laying of
a slightly greater amount on Fourth Street, thus increasing the total
length of the tubes a little. Wrought-iron tube, coated with some
alloy, probably composed largely of tin or zinc, was used for this
extension. The wrought-iron tube is not as good as the bored cast iron.
[Illustration: FIG. 7.
BOURSE BUILDING, PHILADELPHIA.]
[Illustration: FIG. 8.
PNEUMATIC TUBES SUSPENDED IN THE BASEMENT OF THE MAIN POST-OFFICE.]
=Description of the Tubes, Method of Laying, etc.=—This Chestnut
Street line consists of two tubes, one for despatching carriers from
and the other to the main post-office. The distance between the two
stations is two thousand nine hundred and seventy-four feet, requiring
five thousand nine hundred and forty-eight feet of tube. The inside
diameter of the tube is six and one-eighth inches, and it was made in
sections each about eleven feet long, with “bells” cast upon one end,
in order to join the sections with lead and oakum, calked in the usual
manner of making joints in water- and gas-pipes, with this exception,
that at the bottom of the bell a counter-bore was turned to receive
the finished end of the next section. By thus machining the ends of
each section of tube and having them fit accurately together, male and
female, a practically continuous tube was formed with no shoulders
upon the interior to obstruct the smooth passage of the carriers.
Joints made in this way possess another great advantage over flanged
and bolted joints, in that they are slightly yielding without
leaking, and so allow for expansion and contraction due to changes of
temperature. Each joint takes care of the expansion and contraction of
its section, which is very slight, but if all were added together would
amount to a very large movement. Another advantage of the “bell” joint
is that it permits slight bends to be made in the line of tube without
requiring special bent sections. Where short bends had to be made, at
street corners, in entering buildings, and other similar places, brass
tubes were used, bent to a radius of not less than six feet, or about
twelve times the diameter of the tube. (One of the brass bends may be
seen in Fig. 10.) The bends were made of seamless tubing, bent to the
desired form and radius in a hydraulic machine. To prevent them from
being flattened in the process of bending, they were filled with resin,
which was afterwards melted out. Flanges were screwed and soldered
to the ends of the bent brass tubes, and they were bolted to special
flanged sections of the iron tube.
The tubes were laid in the trench and supported by having the ground
firmly tamped about them. Usually one tube was laid above the other,
with an iron bracket between, but frequently this arrangement had to
be departed from in order to avoid obstructions, such as gas- and
water-pipes, sewers, man-holes, etc. The depth of the tubes below the
pavement varied from two to six feet, and in one place, in order to
pass under a sewer, the extreme depth of thirteen feet was reached. At
the street crossings it was frequently difficult to find sufficient
space to lay the tubes. At the intersection of Fifth and Chestnut
Streets a six-inch water-main had to be cut and a bend put in. A
seven-strand electric cable, used for telephoning and signalling,
was laid upon the top of one of the tubes, protected by a strip of
“vulcanized wood,” grooved to fit over the cable. The cable and
protecting strip of wood were fastened to the tube by wrought-iron
straps and bolts.
The tubes enter the main post-office on the Chestnut Street side,
through one of the windows, and are suspended from the ceiling along
the corridor in the basement for a distance of nearly two hundred
feet. Fig. 8 shows the tubes thus suspended. They terminate upon the
ground floor about the centre of the building, and near the cancelling
machines.
[Illustration: FIG. 9.
DUPLEX AIR-COMPRESSOR IN THE BASEMENT OF THE MAIN POST-OFFICE.]
[Illustration: FIG. 10.
TANKS AND TUBE IN THE BASEMENT OF THE MAIN POST-OFFICE.]
=Air-Compressor—Method of Circulating the Air.=—The current of air
that operates the tubes is supplied by a duplex air-compressor
located in the basement of the main post-office. This machine is
shown in Fig. 9, and requires no detailed description, as it does not
differ materially from air-compressors used for other purposes. The
stroke is twenty-four inches, the diameter of the steam-cylinders
ten inches, and the air-cylinders eighteen inches. The air-cylinders
are double acting, with poppet-valves, and have a closed suction.
The speed of the machine varies slightly, being controlled by a
pressure-regulator that maintains a practically constant pressure
in the tank that feeds the tube. The engines develop a little over
thirty horse-power under normal conditions. The pressure of the air
as it leaves the compressor is usually six or seven pounds per square
inch. Compressing the air heats it to about 156° F., but this is not
sufficient to require water-jackets about the air-cylinders. From the
compressor the air flows to a tank, shown on the right in
Fig. 10, where any oil or dirt contained in the air is deposited. The
principal purpose of the tank is, however, to form a cushion to reduce
the pulsations in the air caused by the periodic discharge from the
cylinders of the compressors, and make the current in the tube more
steady. From this tank the air flows to the sending apparatus on the
ground floor of the post-office and thence through the outgoing tube
to the sub-post-office. At the sub-post-office, after flowing through
the receiving and sending apparatus, it enters the return tube and
flows back to the main office, passing through the receiving apparatus
there and then to a tank in the basement,—the left tank in Fig. 10. The
air-compressor draws its supply from this tank, so that the air is used
over and over again. This return tank has an opening to the atmosphere,
which allows air to enter and make up for any leakage or escape at the
sending and receiving apparatus, thereby maintaining the atmospheric
pressure in the discharge end of the tube and in the suction of the
compressor. The tank serves to catch any moisture and dirt that come
out of the tube. Fig. 11 is a diagram showing the direction and course
of the air-current. It will be noticed that both the out-going and
return tube are operated by _pressure_, in distinction from _exhaust_.
The air is forced around the circuit by the air-compressor. There is no
exhausting from the return tube. The pressure of the air when it enters
the tube at the main post-office is, say, seven pounds per square inch;
when it arrives at the sub-post-office the pressure is about three and
three-quarters pounds, and when it gets back to the main office and
enters the return tank, the pressure is zero or atmospheric. Thus it
will be seen that the pressure becomes less and less as the air flows
along the tube. This is not the pressure that moves the carriers, but
the pressure of the air in the tube, a pressure that exists when there
are no carriers in the tube. It is the pressure that would be indicated
if you should drill a hole into the tube and attach a gauge.
[Illustration: FIG. 11.]
=Terminal Apparatus.=—When the construction of this line was begun, it
was the intention of the Pneumatic Transit Company to use the apparatus
of the Electro-Pneumatic Transit Company, at both stations, for sending
and receiving carriers, and so-called working-drawings were obtained
for this purpose. The sending apparatus was constructed according to
the designs furnished, but, upon examination of the drawings of the
receiving apparatus, it was so apparent that it would not work as
intended that it was never constructed.
The writer was asked to design an automatic receiver to stop the
carriers without shock upon their arrival at the stations, and to
deliver them upon a table without appreciable escape of air,—something
that would answer the requirements of the present plant.
[Illustration: FIG. 12.
_TRANSMITTER.—PHILA._
SENDING APPARATUS.]
=The Sender.=—The sending apparatus is for the purpose of enabling
the operator to place a carrier in the tube without allowing the air
to escape. In other words, it is a means of despatching carriers. The
apparatus for this purpose, already referred to, is simply a valve. A
side view and section of it are shown in Fig. 12. Fig. 15 is a view of
the apparatus in the main post-office. The sending apparatus is seen
on the left. Fig. 13 is a view of the sub-post-office apparatus, and
here a man may be seen in the act of despatching a carrier. Referring
to the section, Fig. 12, it will be seen that the sending apparatus
consists of a short section of tube supported on trunnions and enclosed
in a circular box. Normally this short section of tube stands in line
with the main tube, and the air-current passes directly through it.
It is shown in this position in the figure. When a carrier is to be
despatched, this short section of tube is rotated by a handle until
one end comes into coincidence with an opening in the side of the box.
In this position the air flows through the box around the movable
tube. A carrier can then be placed in the short section of tube and be
rotated by the handle into line with the main tube. The carrier will
then be carried along with the current of air. A circular plate covers
the opening in the box where the carrier is inserted when the sending
apparatus is closed.
At the sub-post-office this sending apparatus is placed in a horizontal
position, but its operation is the same.
[Illustration: FIG. 13.
RECEIVING AND SENDING APPARATUS IN THE SUB-POST-OFFICE.]
[Illustration: FIG. 14.
_APPARATUS AT SUB-STATION—PHILA._]
[Illustration: FIG. 15.
TERMINALS OF THE TUBE IN THE MAIN POST-OFFICE.]
=Sub-Post-Office Receiver.=—We have already explained that the
air-pressure in the tube at the sub-post-office is about three and
three-quarters pounds per square inch. With such a pressure we
cannot open the tube to allow the carriers to come out. They must
be received in a chamber that can be closed to the tube after the
arrival of a carrier and then opened to the atmosphere. Furthermore,
this chamber must act as an air-cushion to check the momentum of the
carriers. Fig. 13 shows the sub-post-office apparatus when a carrier
is being delivered from the receiving apparatus, or, as we will name
it for convenience, the receiver. Fig. 14 is a drawing of the same
apparatus, partly in section, that shows more clearly its method
of operation. This drawing shows the sending apparatus in a
different position from Fig. 13, but that is immaterial. The receiver
consists of a movable section of tube, about twice the length of a
carrier, closed at one end, supported upon trunnions, and normally
in a position to form a continuation of the main tube from which the
carriers are received. When a carrier arrives it runs directly into
the receiver, which being closed at the end forms an air-cushion
that stops the carrier without shock or injury. Just before reaching
the receiving chamber the current of air passes out through slots in
the walls of the tube into a jacket that conducts it to the sending
apparatus, as shown in Fig. 14. At the closed end of the receiving
chamber, or air-cushion, is a relief valve, normally held closed by
a spring. As the carrier compresses the air in front of it, this
valve opens and allows some of the air to escape, which prevents the
carrier from rebounding into the tube. Under the outer end of the
receiving chamber is a vertical cylinder, E, Fig. 14, supported upon
the base-plate containing a piston. The piston of this cylinder is
connected by a piston- and connecting-rod to the receiving chamber.
When air is admitted to the cylinder under the piston, the latter rises
and tilts the receiving chamber to an angle of about forty degrees,
which allows the carrier to slide out. The receiving chamber carries
a circular plate, C, that covers the end of the main tube when it is
tilted. A small piston slide-valve, F, located near the trunnion of
the receiving chamber, controls the admission and discharge of air to
and from the cylinder E, upon the arrival of a carrier. When a carrier
arrives and compresses the air in the air-cushion or receiving
chamber, a small portion of this compressed air is forced through pipe
G, to a small cylinder containing a piston and located just above the
piston slide-valve F. The increased pressure acting on the piston moves
it downward, and it in turn moves the slide-valve F. Thus it will be
seen that the stopping of the carrier causes the receiving chamber to
be tilted and the carrier slides out on to an inclined platform, K.
This platform is hinged at one end, and supported at the angle seen
in the figure by a counterweight. When a carrier rests upon it, the
weight of the carrier is sufficient to bear it down into a horizontal
position; in this position the carrier rolls off on to a table or
shelf. The platform, K, is connected by rods, bell-cranks, etc., to the
piston slide-valve, so that when it swings downward by the weight of
a carrier, the slide-valve is moved upward into its normal position,
and this causes the receiving chamber to tilt back into a horizontal
position ready to receive the next carrier. The time that elapses from
the arrival of a carrier until the receiving chamber has returned to
its horizontal position is not more than three or four seconds. Nothing
could operate in a more satisfactory manner.
=Main Post-Office Receiver.=—At the main post-office we have a
receiver of a different type. It will be remembered that the pressure
in the return tube at the main post-office is nearly down to zero or
atmospheric, so that we can open the tube to allow the carriers to pass
out without noise or an annoying blast of air. Figs. 15 and 16 show
the main-office apparatus, and Fig. 17 is a drawing of the same. Here
the receiver consists of a section of tube closed by a sluice-gate,
located at B, Fig. 17. The air-current passes out through slots in the
tube into a branch pipe leading to the return tank in the basement.
These slots are located about four feet back of the sluice-gate, so
that the portion of the tube between the slots and the sluice-gate
forms an air-cushion to check the momentum of the carriers. The
sluice-gate is raised and lowered by a piston moving in a cylinder
located just above the gate. The movement of this piston is controlled
by a piston slide-valve in a manner similar to the apparatus at the
sub-post-office. Air for operating the piston is conveyed through the
pipe D, Fig. 17, from the pipe leading from the air-compressor to the
sending apparatus. This air is at about seven pounds pressure per
square inch.
[Illustration: FIG. 16.
RECEIVING APPARATUS AT THE MAIN POST-OFFICE.]
[Illustration: FIG. 17.
_APPARATUS AT THE MAIN OFFICE—PHILA._]
When a carrier arrives, after passing the slots that allow the
air-current to flow into the branch pipe, it compresses the air in
front of it against the gate. This compression checks its momentum,
and it comes gradually to rest. The air compressed between the
carrier and the sluice-gate operates to move the piston slide-valve,
thereby admitting air to the gate cylinder under the piston, which
rises, carrying with it the sluice-gate. The tube is now open to the
atmosphere, and there is just sufficient pressure in the tube to push
the carrier out on to a table arranged to receive it. As the carrier
passes out of the tube it lifts a finger out of its path. This finger
is located at E, Fig. 17, and when it is lifted by the passing carrier
it moves the piston slide-valve, and the sluice-gate is closed. A valve
is located in the branch-pipe that conducts the air to the return tank
in the basement. If the pressure in the tube is not sufficient to
push the carrier out on to the table, this valve is partially closed,
thereby increasing the pressure to a desired amount.
[Illustration: FIG. 18.
CARRIER.]
[Illustration: FIG. 19.
CARRIER.]
=The Carrier.=—We have frequently spoken of the carrier, which contains
the mail and other parcels that are transported from one office to
the other. In Fig. 13, showing the sub-post-office apparatus, we see
one of these carriers being despatched by the attendant and another
being delivered from the tube. In Fig. 15 several carriers may be
seen standing on the floor. Fig. 18 shows a carrier with the lid
open, ready to receive a charge of mail, and Fig. 19 shows the same
closed, ready for despatching. The construction of the carrier is
shown by the drawing, Fig. 20. The body of the carrier is steel,
about one-thirty-second of an inch in thickness. It is made from a
flat sheet, bent into a cylinder, riveted, and soldered. The
length outside is eighteen inches, and the inside diameter is five and
one-quarter inches. The front end is made of a convex disk of steel,
stamped in the desired form, and secured to the body of the carrier
by rivets, with the convex side inward. It is necessary to have a
buffer upon the front end of the carrier to protect it from blows
that it might receive, and this buffer is made by filling the concave
side of the front head with felt, held in place by a disk of leather
and a central bolt. The leather disk is made of two pieces, riveted
together, with a steel washer between. The steel washer is attached
to the head of the bolt. The carrier is supported in the tube on two
bearing-rings, located on the body of the carrier a short distance from
each end. The location of these rings is so chosen that it permits a
carrier of maximum length to pass through a bend in the tube of minimum
radius without becoming wedged. This is a very important feature in the
construction of carriers, but does not appear to have been utilized in
other systems.
[Illustration: FIG. 20.
_MAIL CARRIER.—PHILA._]
The bearing-rings are made of fibrous woven material, especially
prepared, and held in place by being clamped between two metal rings,
one of which is riveted to the body of the carrier. Of course these
rings wear out and have to be replaced occasionally, but their usual
life is about one thousand miles. The rear end of the carrier is closed
by a hinged lid and secured by a special lock. The lock consists of
three radial bolts that pass through the body of the carrier and the
rim of the lid. These bolts are thrown by three cams, attached to a
short shaft that passes through the lid and has a handle or lever
attached to it upon the outside of the lid. This cam-shaft is located
out of the geometrical centre of the lid in such a position that when
the lever or handle is swung around in the unlocked position, it
projects beyond the periphery of the lid, and in this position the
carrier will not enter the tube. When the lid is closed and locked,
the lever lies across the lid in the position shown in Fig. 19, and
when the carrier is in the tube it cannot become unlocked, for the
lever cannot swing around without coming in contact with the wall of
the tube. This insures against the possibility of the carriers opening
during transit through the tube. The empty carriers weigh about nine
pounds, and when filled with mail, from twelve to fifteen pounds. They
have a capacity for two hundred ordinary letters, packed in the usual
manner.
=Operation of the Tubes.=—The tubes are kept in constant operation
during the day, and six days of the week. The air-compressor is started
at nine o’clock in the morning and runs until seven in the evening,
except during the noon hour, the air flowing in a constant steady
current through the tubes. When a carrier is placed in the tube it is
carried along in the current without appreciably affecting the load on
the compressor. Carriers may be despatched at six-second intervals,
and when they are despatched thus frequently at each office, there will
be eighteen carriers in the tube at the same time. If ten carriers per
minute are despatched from each office, and each carrier contains two
hundred letters, the tube has a carrying capacity of two hundred and
forty thousand letters per hour, which is far beyond the requirements
of this office. About five hundred carriers a day are despatched
from each office. This varies considerably on different days and at
different seasons of the year. Experience has taught that a certain
period of time should elapse between the despatching of carriers, in
order that they may not come in contact with each other, and that the
receivers may have time to act. With the present plant this period is
made about six seconds. In order to make it impossible for carriers
to be despatched more frequently than this, time-locks are attached
to the sending apparatus. One of these locks may be seen in Fig. 13,
connected to the handle of the sending apparatus. It is so arranged
that when a carrier is despatched a weight is raised and allowed to
fall, carrying with it a piston in a cylinder filled with oil. While
the weight is rising and falling the sending apparatus is locked, but
becomes unlocked when the weight is all the way down. A by-pass in the
cylinder permits the oil to flow from one side of the piston to the
other, and the size of this by-pass can be regulated, thus determining
the time that the weight shall take in descending. This makes a simple
and effective time-lock that does not get out of order.
The time required for a carrier to travel from the main to the
sub-post-office is sixty seconds, and from the sub- to the main
post-office, fifty-five seconds. This difference of time in going and
returning is due to the expansion of the air in the tube, as will
be explained more fully in another place. The distance between the
offices being two thousand nine hundred and seventy-four feet, gives
an average speed of about fifty-two feet per second, or 35.27 miles
per hour. Of course the speed can be increased by increasing the
air-pressure, but this speed is found in practice to be ample for all
requirements. In order to give some idea of the energy possessed by
one of these carriers travelling at this speed, it may be said that
if the end of the tube were left open and turned upward, an emerging
carrier would rise about forty feet into the air. It is easy to imagine
how apparatus, depending for its operation upon impact with a moving
carrier, would be soon destroyed, as well as the carriers themselves.
This is why receiving apparatus used with small tubes and light
carriers cannot be applied to large tubes with heavy carriers.
No serious trouble has ever been experienced from carriers getting
wedged in any part of these tubes.
=Benefits of the System.=—The advantages to the post-office department
by the adoption of this system have been numerous, and the post-office
officials who are familiar with the operation of the tubes frequently
speak in high terms of their usefulness. Formerly the mail was
transported from one office to the other by a wagon making a trip
every half-hour. Considerable time has been saved by the greater speed
of transit, but even more time is gained by keeping the mail moving
instead of allowing it to accumulate and then despatching it in bulk.
With the pneumatic system a letter posted in the sub-post-office will
reach its destination just as quickly as if posted at the main office,
and sometimes more quickly. Let us take an example, first, with the old
wagon service. Suppose that you drop a letter in the sub-post-office;
it lies there, say, fifteen minutes waiting for the departure of the
next wagon; it is put into a pouch with hundreds of other letters, and
ten minutes are consumed in transporting it to the main office. When
it arrives there the pouch is thrown on the floor at the entrance of
the building; in a few minutes, more or less, a clerk takes the pouch,
throws it on a truck and wheels it around to the cancelling machines,
where it may lie for five or ten minutes more before being opened,
and then perhaps five minutes will elapse before your letter reaches
the cancelling machine. It would not be unusual for three-quarters
of an hour to elapse from the time you dropped your letter in the
office until it was cancelled. Now with the pneumatic tube service
forty minutes of this time will be saved; for immediately after you
drop your letter in the office it will be despatched through the tube
and delivered on the table in front of the cancelling machines. Soon
after the tubes were installed the postmaster’s attention was called
to an instance where letters from the sub-office were sent through
the tube and were despatched to New York City one train earlier than
they could have been had the old wagon service been in use. People
frequently post letters requesting that they be sent through the tube;
of course they would be sent in that way if the request was not made,
but it shows that the public recognize the better service. Formerly
mail was collected from the street boxes in the banking section of the
city and the collectors carried it to the main office. After the tubes
were installed this mail was carried to the sub-post-office to be sent
through the tube, and the time formerly occupied in walking to the main
office was then utilized in having the men face up the letters ready
for the cancelling machines,—a double saving in time besides making
their labor much lighter and enabling them to do more useful work.
Since the sub-office has been established in the Bourse, it has been
made a distributing as well as receiving office. At least two more
deliveries of mail are made each day in the Bourse building than in any
other office building in the city.
All letters mailed in the sub-office with a special delivery stamp are
despatched through the tube immediately.
It is now nearly four years since the system was put into operation.
During that time more than thirty-five million letters have been
transported, and all the repairs to the system have not required it
to be stopped for more than a few hours. During the first year the
Pneumatic Transit Company operated the tubes at their own expense,
agreeing at the end of that time to take them out if the government so
requested. Since the first year the government has paid the running
expenses.
Such is the history of the first United States pneumatic postal system.
Such is the history of the first pneumatic tubes of sufficient size to
carry all the first class and most of the lower classes of mail, in
this or any other country, so far as the writer knows.