Transcriber’s Notes:
The spelling, punctuation and hyphenation are as the original, with the
exception of apparent typographical errors which have been corrected.
Italic text is denoted _thus_.
Bold text is denoted =thus=.
[Illustration: THE MAIN POST-OFFICE, PHILADELPHIA.]
THE
PNEUMATIC DESPATCH TUBE SYSTEM
OF THE
BATCHELLER PNEUMATIC TUBE CO.
ALSO
FACTS AND GENERAL INFORMATION RELATING TO PNEUMATIC DESPATCH TUBES
BY B. C. BATCHELLER, B.Sc. MECHANICAL ENGINEER
[Illustration]
PHILADELPHIA PRESS OF J. B. LIPPINCOTT COMPANY 1897
COPYRIGHT, 1896,
BY
B. C. BATCHELLER.
CONTENTS.
CHAPTER I.
A BRIEF HISTORICAL SKETCH. PAGE
EARLY RECORDS 9
PRACTICAL BEGINNING OF THE ART—THE LONDON PNEUMATIC TELEGRAPH 10
THE SIEMENS CIRCUIT SYSTEM 14
RECENT IMPROVEMENTS IN THE LONDON SYSTEM 16
AN UNDERGROUND PNEUMATIC RAILWAY FOR TRANSPORTATION OF MAIL 19
THE BERLIN PNEUMATIC TELEGRAPH 20
THE PARIS PNEUMATIC TELEGRAPH 22
THE PNEUMATIC TELEGRAPH OF OTHER CITIES 25
PNEUMATIC TUBES IN AMERICA 25
CHAPTER II.
THE PNEUMATIC TRANSIT COMPANY AND THE FIRST PNEUMATIC TUBES FOR THE
TRANSPORTATION OF UNITED STATES MAIL.
ORGANIZATION 28
AIM AND OBJECT OF THE COMPANY 28
THE CLAY-LIEB PATENTS 30
FRANCHISES AND FIRST GOVERNMENT CONTRACT 33
SEARCH FOR TUBES 34
METHOD OF MANUFACTURING TUBES 35
LAYING AND OPENING THE TUBES FOR TRAFFIC 37
DESCRIPTION OF THE TUBES, METHOD OF LAYING, ETC. 38
THE AIR-COMPRESSOR—METHOD OF CIRCULATING THE AIR 40
TERMINAL APPARATUS 42
THE SENDER 43
SUB-POST-OFFICE RECEIVER 44
MAIN POST-OFFICE RECEIVER 47
THE CARRIER 50
OPERATION OF THE TUBES 52
BENEFITS OF THE SYSTEM 54
CHAPTER III.
THE SYSTEM AND APPARATUS OF THE BATCHELLER PNEUMATIC TUBE COMPANY.
GENERAL ARRANGEMENT AND ADAPTABILITY OF THE SYSTEM 57
THE SIZE OF TUBES 64
SYSTEM OF VERY LARGE TUBES 65
GENERAL ARRANGEMENT OF APPARATUS IN THE STATIONS—TWO-STATION,
TWO-COMPRESSOR LINE 69
TWO-STATION, ONE-COMPRESSOR LINE 72
THREE- TO EIGHT-STATION LINE 74
SENDING APPARATUS 79
SENDING TIME-LOCK 84
INTERMEDIATE STATION TIME-LOCK 88
ELECTRO-PNEUMATIC CIRCUIT-CLOSER 91
THE OPEN RECEIVER 94
THE CLOSED RECEIVER 99
THE INTERMEDIATE STATION RECEIVING AND TRANSFER APPARATUS 106
CARRIERS 115
AIR SUPPLY 117
FANS 117
BLOWERS 117
AIR COMPRESSORS 118
THE TUBE, LINE CONSTRUCTION, ETC. 122
CHAPTER IV.
FACTS AND GENERAL INFORMATION RELATING TO PNEUMATIC TUBES.
DEFINITIONS 124
INTERMITTENT AND CONSTANT AIR-CURRENT 125
LAWS GOVERNING THE FLOW OF AIR IN LONG TUBES 126
LAW OF PRESSURE 128
USES OF PRESSURE CURVES 130
LAW OF VELOCITY 130
CHARACTERISTICS OF THE VELOCITY CURVE 132
USES OF VELOCITY CURVES 133
QUANTITY OF AIR USED 134
TEMPERATURE OF THE AIR 135
HORSE-POWER 136
EFFICIENCY 137
PRESSURE AND EXHAUST SYSTEMS 138
LAWS EXPRESSED IN MATHEMATICAL FORMULÆ 141
MOISTURE IN THE TUBES 142
LOCATION OF OBSTRUCTIONS IN TUBES 143
PREFACE.
I have been prompted to prepare this book by the frequent inquiries
made regarding the details of our system of pneumatic tubes. These
inquiries have come from people interested in our company, from
others interested in companies that have purchased the right to use
our apparatus, from people desirous of becoming interested in a
pneumatic-tube business, from would-be purchasers of pneumatic tubes,
and from people interested in pneumatic tubes from a scientific,
engineering, or mechanical point of view. This book is not intended to
be a treatise on pneumatic tubes. In the first chapter I have given
a brief sketch of what has been done in the application of pneumatic
tubes from the earliest records to the present time. The second chapter
contains a description of the postal tubes in Philadelphia, and the
third chapter describes our system in detail. Following this is a short
chapter explaining the theory of pneumatic tubes, or the theory of
the flow of air in long pipes, stating the more interesting facts and
relations in as plain and simple a manner as possible. Mathematical
formulæ have been purposely avoided.
Several plates showing the Philadelphia postal line have been kindly
loaned to me by the Engineers’ Club of Philadelphia. They formerly
appeared in a paper read by Mr. A. Falkenau before that club. I
have also to thank the and Drill Co., the B. F. Sturtevant Co.,
the Wilbraham-Baker Blower Co., and J. B. Stewart for the use of
electrotypes of their machines.
B. C. B.
October 6, 1896.
THE
BATCHELLER PNEUMATIC TUBE SYSTEM.
CHAPTER I.
A BRIEF HISTORICAL SKETCH.
=Early Records.=—The earliest reference to pneumatic transmission of
which we find any record is a paper presented to the Royal Society of
London, by Denis Papin, in the year 1667, entitled “Double Pneumatic
Pump.” His plan was to exhaust the air from a long metal tube by two
large cylinders. The tube was to contain a piston, to which a carriage
was attached by means of a cord. The “American Cyclopædia” goes on to
say, “More than a century elapsed before any further effort in this
direction was made. Paucbrouke’s ‘Dictionnaire Encyclopédique des
Amusements des Sciences’ (1792) gives a description of a machine by
M. Van Estin, by means of which a hollow ball holding a small package
was propelled by a blast of air through a tube several hundred feet in
length, and having many curves. This plan seems, however, to have been
more an amusement than an attempt to introduce an industrial scheme.
With more regard to practical results, Medhurst, an engineer of London,
published a pamphlet on the subject in 1810. He proposed to move small
carriages on rails in air-tight tubes or tunnels, by compressed air
behind, or by creating a partial vacuum in front. In 1812 he published
another pamphlet; but the plan was not put into successful operation,
principally from insufficient means of exhaustion. About 1832 he
proposed to connect the carriage inside such a tube with a passenger
carriage running on the top of the tube; and, although the latter
project has never been commercially successful, it was the first to
be practically attempted. More than a score of patents were taken out
on the Continent and in England and America, none of which met with
any practical success. Returning to the original idea of Denis Papin,
inventors attempted to accomplish pneumatic transmission by moving the
load inside the tube, and in course of time achieved success. In France
MM. Jarroux and Taisseau presented a project for atmospheric telegraphy
before the Academy of Sciences, and they were succeeded in the same
direction by MM. Brochet and Ardor.”
=Practical Beginning of the Art. The London Pneumatic
Telegraph.=—London has the honor of being the first city to have
a practical system of pneumatic telegraphy. The first tubes were
installed by the Electric and International Telegraph Company, the work
being planned and carried out by their engineer, Mr. Josiah Latimer
Clark, in 1853 and 1854. The first tube to be laid was one and one-half
inches in diameter, and extended from the central station, Founder’s
Court, Lothbury, to the Stock Exchange, Throgmorton Street, a distance
of two hundred and twenty yards. The tube was operated intermittently
by connecting it to a vacuum chamber at the central station. Carriers
were sent only in one direction. A steam-pump was used to maintain the
vacuum. Much experience was gained from the use of this first tube. In
1858 some improvements were made by Mr. C. F. Varley, and I can best
describe them by quoting from the discussion of Mr. Carl Siemens’s
paper on “Pneumatic Despatch Tubes: The Circuit System” before the
Institution of Civil Engineers, as recorded in the minutes of that
society. “Later, about the year 1858, when a pipe two and one-fourth
inches internal diameter was extended from Telegraph Street to Mincing
Lane, thirteen hundred and forty yards in length, the traffic was so
considerable that it was found desirable to have the power of sending
messages in both directions. To effect that a smaller pipe, one and
one-half inches in internal diameter, was laid between Telegraph
Street and Mincing Lane, with a view to carrying the vacuum to the
latter station, so as to take messages in the opposite direction. This
smaller pipe was found to so wiredraw the current that the pipe would
not work, the leakage past the carrier being too considerable; and
accordingly a large chamber was built in the basement floor or kitchen
at the corner of Mincing Lane and Leadenhall Street to collect power
or vacuum for bringing the messages from Telegraph Street to Mincing
Lane. This chamber was constructed of timber, fourteen feet by twelve
feet broad and ten feet high, and was covered with lead. It was not
strong enough to withstand the pressure; for one day, a carrier having
stuck half-way, and when there was a higher vacuum than usual,—viz.,
twenty-three inches of mercury,—it collapsed with a loud report. At
the time the landlord of the house happened to be dining in the next
room, and he suddenly found himself, his table, dinner, and the door,
which was wrenched off its hinges, precipitated into the room amongst
the _débris_ of the chamber. The windows were forced inwards, and those
on the opposite side of Mincing Lane and Leadenhall Street were drawn
outwards. The damage was considerable. This accident put an end, for a
time, to the attempt to send telegraph messages by means of a vacuum
conveyed through this smaller pipe. About that time he (Mr. Varley)
became the engineer-in-chief of the Electric Telegraph Company, and
got permission from the directors to introduce a new system,—viz.,
compressed air,—though many persons contended that it would be
impossible to blow messages through a pipe, because all attempts to
blow air through long pipes had utterly failed; while others said
that, if messages were sent, they would go much slower than with the
vacuum.... In his (Mr. Varley’s) apparatus, for he was the first to
introduce compressed air, the reverse was found to be the case, and for
this reason: the tube did not consume power until a message was about
to be forwarded; and in a tube thirteen hundred yards in length, and
two and one-fourth inches in diameter, fifteen seconds elapsed before
the vacuum was felt at the distant end after communication had been
established with the exhausted chamber at the engine end of the tube,
consequently the carrier did not start until after fifteen seconds
had elapsed. When a message was sent by compressed air, it was sent
from the end at which the power was applied, and the carrier started
at once, thus gaining a start of fifteen seconds; now, inasmuch as
the air in the tube had to be compressed, it started at a very high
velocity, and when it reached the other end the compressed air in
expanding gave it a higher velocity. The result was, in thirteen
hundred and forty yards, from Telegraph Street to Mincing Lane,
the carriers were drawn by vacuum, on an average, in from sixty to
seventy seconds, and were propelled by compressed air in about fifty
or fifty-five seconds, the difference of pressure in each case being
nearly equal.”
The first one and one-half inch tubes laid under the direction of Mr.
Clark were of iron with screwed joints. They gave much trouble from
roughness upon the interior, which wore the carriers very rapidly, and
from water that was drawn in through leaky joints. When the extensions
were made in 1858 and afterwards, two and one-fourth inch lead tubes
were used with plumber’s joints made over a heated mandrel, which made
the joints very smooth upon the interior. The carriers were of gutta
percha in the form of a cylinder closed at one end and fitted with a
cap at the other. The outside was covered with felt or drugget.
When a carrier was to be despatched, a signal was sent to an attendant
at the pump end of the tube, who closed that end and connected the tube
to an exhausted chamber by opening a valve. As soon as the carrier
arrived, he closed the valve and opened the tube, which allowed the
carrier to drop out. Mr. Varley improved the method of operating the
valves by making the air pressure do the work by means of cylinders
and pistons when the attendant pressed a button. He also improved the
carriers by doing away with the cap and using in its place an elastic
band to hold the messages in place.
We have seen that Mr. Clark designed the first tube used in connection
with the telegraph, and that it was a single tube, operated in one
direction only by vacuum, being operated only when there were messages
to send. This was extended and improved by Mr. Varley, who increased
the diameter of the tubes from one and one-half inches to two and
one-quarter inches, and operated them in both directions, using vacuum
for sending in one direction and compressed air for sending in the
other. The air current was maintained in the tubes only when messages
were sent.
Great credit is due to Sir Rowland Hill, who, in 1855, had a proposed
method of conveying mails in the city of London, through nine- and
thirteen-inch tubes, thoroughly investigated. It was decided at this
time that the saving in time over that consumed by mail carts would not
warrant the expense of installing such a system.
=The Siemens Circuit System.=—The next progressive step was made
by Siemens Brothers, of Berlin, who proposed a new system called
the “circuit system,” in which two tubes were used, the up tube
being connected to the down tube at the distant end. The air was
compressed into one end of the circuit and exhausted from the other,
and, furthermore, it was kept in constant circulation. Carriers were
despatched by inserting them into the air-current without stopping it,
in one direction in one tube or in the opposite direction in the other.
Another feature of the Siemens system was the placing of three or more
stations on one double line of tubes. Carriers could be stopped at an
intermediate station by inserting in the tube an obstructing screen
which the air would pass but which would stop a carrier. This system
is described in detail in a paper read by Mr. Carl Siemens before the
Institution of Civil Engineers, London, November 14, 1871, Vol. XXXIII.
of the Proceedings. The Siemens apparatus for sending and receiving
carriers consisted of two short sections of tube attached to a rocking
frame so that either could be swung by hand into line with the main
tube. One of the tube sections was open at both ends, and was used for
despatching carriers. A carrier was placed in it, then it was swung
into line with the main tube, when the air-current passing through
swept the carrier along. The other tube section contained a perforated
screen in one end and was used to receive carriers. When it was in line
with the main tube and a carrier arrived, the carrier was stopped by
striking the screen, then the tube section was swung to one side and
the carrier pushed out with a rod. A by-pass was provided for the air
around the apparatus so that its flow was not checked when the tube
section was swung. When a carrier was despatched to an intermediate
station, a signal was sent, and then the section of tube containing
the screen was interposed in the line of the tube to stop the carrier
upon its arrival. The carriers used by Mr. Siemens were made of
gutta-percha, papier maché, or tin, closed at one end and fitted
with a cover at the other. They were covered with felt, drugget, or
leather. The front ends of the carriers were provided with thick disks
of drugget or leather fitting the tube loosely, and the opposite ends
were surrounded with pieces of the same material attached to them like
the leather of an ordinary lifting pump.
In 1869 Messrs. Siemens Bros. received an order from the British
government to install an experimental line of tubes between the central
telegraph station and the general post-office. This was completed in
1870, and after a half-year’s test it was extended to Fleet Street,
and finally to Charing Cross. The tubes were of iron, three inches
in diameter, with flanged and bolted joints. It was found, after
some experience, that there was no advantage in the circuit, so the
up and down tubes were separated at Charing Cross Station and worked
independently.
=Recent Improvements in the London System.=—In 1870 Mr. J. W. Wilmot
designed a double sluice-valve by means of which carriers could be
despatched continuously without stopping the flow of air in the tubes.
Mr. Wilmot further increased the working capacity of pneumatic tubes
when, in 1880, he invented an intermediate automatic signaller, by
means of which a carrier signals the passage of a given point on its
journey, showing that the section of the tube traversed is clear, thus
allowing a second carrier to be despatched before the first has reached
its destination.
[Illustration: FIG. 2.
DIAGRAM ILLUSTRATING THE PNEUMATIC TUBE SYSTEM LONDON RADIAL SYSTEM.
LONDON RADIAL SYSTEM.]
From this beginning the English system developed into what has been
termed a “radial system;” that is to say, one principal and several
minor central pumping stations have been established, and from these
radiate tubes to numerous sub-stations (see Fig. 2). Some of the
stations are connected with double lines for sending in opposite
directions. The out-going tube from the pumping station is worked by
compressed air, and the incoming tube by exhaustion. Other stations
are connected by single tubes, and they are operated alternately by
compression and exhaustion. Intermediate stations are located on some
of the lines. For the central station the Varley valves were found
too expensive and troublesome to keep in order, so they were replaced
by the Wilmot double sluice-valves, which are operated manually. In
recent years the sluice-valves have been in turn replaced by what are
termed D-boxes, a simpler form of apparatus. At the sub-stations the
tube terminates in a box into which the carriers drop. As the system
has been gradually extended, tubes two and three-sixteenths inches
inside diameter have been used for short lines, and three-inch tubes
for long lines. The tubes are of lead laid inside a cast-iron pipe
which serves as a shield, protecting them from injury. They are laid
in twenty-nine foot sections, the joints being made by soldering over
a steel mandrel, which is afterwards drawn out by a chain. The joints
in the cast-iron protecting pipe are made by caulking with yarn and
lead. “Electric signals are used between the central and sub-stations,
consisting of a single stroke bell and indicator, giving notice of
the arrival and departure of carriers, and to answer the necessary
questions required in working. Where there are intermediate stations
the tubes are worked on the block system, as if it were a railway.
Experience shows that, where great exactness of manipulation cannot
be obtained, it is necessary to allow only one train in each section
of a tube, whether worked by vacuum or pressure. But where there is
no intermediate station, and where the tube can be carefully worked,
carriers may be allowed to follow one another at short intervals in
a tube worked by vacuum, although it is not perfectly safe to do so
in one worked by pressure. In working by pressure it has been found
that, notwithstanding a fair interval may be allowed, carriers are
apt to overtake one another, for no two carriers travel in the same
times, because of differences in fit, unless they are placed end to
end. If signalling be neglected and a carrier happens to stick fast,
being followed by several others, a block will ensue which it will be
difficult to clear, while the single carrier could readily have been
dislodged.” (_Proceedings Institute of Civil Engineers, London_, Vol.
XLIII. p. 61.)
No changes have been made in the carriers from those used in the early
experiments which have already been described.
The London system has grown until it now includes no less than
forty-two stations and thirty-four miles of tubes. Similar systems
have been established in connection with the telegraph in Liverpool,
Manchester, Birmingham, Glasgow, Dublin, and New Castle. The tubes
give a cheaper and more rapid means of despatching telegrams between
sub-stations and central stations than transmission by telegraph, and
local telegrams can be delivered in the sender’s handwriting.
=An Underground Pneumatic Railway for Transportation of Mail.=—Before
describing the systems used in the cities on the Continent of Europe,
we must notice a very large pneumatic tube, or more properly called a
pneumatic tunnel railway, constructed in London for the transportation
of mail from one of the railway stations. The first railway of this
type was constructed in 1863 by the Pneumatic Despatch Company of
London, and extended from Euston to the district post-office in
Eversholt Street, a distance of about eighteen hundred feet. The
tunnel was flat on the bottom, having a D-shaped cross-section two
feet eight inches by two feet eight inches. The carriers or carriages
were cradle-like boxes fitting the tunnel, and they moved at a speed
of seventeen miles per hour, carrying fifteen mails daily. In 1872
two similar but larger tunnels were built from Euston Station to the
general post-office, a distance of fourteen thousand two hundred and
four feet, or two and three-quarters miles. One was for the up traffic,
and the other for the down. The tunnels were four and a half feet
wide by four feet high, the straight part being built of cast iron
and the bends of brick. The line was operated by a fan twenty-two
feet in diameter, which forced the air into one tunnel and exhausted
it from the other, producing a vacuum of ten inches of water, or six
ounces per square inch. The carriages occupied twelve minutes in
traversing the tunnels, and there was one gradient of one to fourteen.
The carriages were ten feet four inches long and weighed twenty-two
hundredweight. “The system was able to transport over the whole line,
allowing for delays, an average of a ton per minute.” The system was
used to transport the mails in bulk, but it was found to be slow and
unsatisfactory, and was very soon abandoned.
=The Berlin Pneumatic Telegraph.=—In 1863 the Prussian government
applied to the firm of Siemens and Halske, of Berlin, for a proposition
to establish a system of pneumatic tubes in that city for the
transmission of telegraph messages. A proposition was accordingly
submitted, and the work was completed in 1865. This first line
consisted of two parallel wrought-iron tubes, two and one-half inches
in diameter, one tube being used exclusively to send in one direction,
and the other in the opposite direction. They extended from the
telegraph station to the Exchange, requiring a total length of five
thousand six hundred and seventy feet of tube. The two tubes were
looped together at the Exchange, and a continuous current of air was
made to circulate in them by a double-acting steam air-pump, located
at the telegraph station. Air was compressed into one end of the tube
and exhausted from the other. With nine inches of mercury pressure and
vacuum the passage was made in ninety-five seconds to the Exchange,
and seventy-five seconds from the Exchange. It was similar to the
line established in London by the same firm some years later, which we
have already described, except that there was no intermediate station.
After the line had been in use for a year and a half, the Prussian
government had it extended, first, from the telegraph station to the
Potsdam gate, with an intermediate station at the Brandenburg gate.
After these preliminary experiments, further extensions were made until
a net-work of tubes extended over the city of Berlin, including no
less than thirty-eight stations and over twenty-eight miles of tubes;
but in laying down this net-work a departure was made from the Siemens
system. Air was no longer kept constantly circulating, but power was
stored up in large tanks, some being exhausted and others filled with
compressed air, which was used when required to send messages, usually
at intervals of five or fifteen minutes. The exhausted tanks were
permanently connected with the closed tubes, which were opened when
required for use. The tanks containing compressed air were connected
to the tubes when messages were sent. The internal diameter of the
tubes was 2.559 inches. They were laid in circuits, including several
stations in a circuit, and the carriers travelled only in one direction
around the circuit. Some outlying stations were connected by a single
tube with central pumping-stations, these single tubes being worked in
both directions. Years of experience have shown the disadvantages of
this circuit-system, and it has gradually been changed to the radial
system, such as is used in London, until now nearly all the stations
are grouped around the central pumping-stations, to which they are
connected directly by radiating tubes. The Siemens apparatus has been
replaced by simpler and less expensive valves and receiving-boxes, the
latest form of which was designed and patented by Mr. Josef Wildemann.
[Illustration: FIG. 3.
DIAGRAM OF PART OF PARIS PNEUMATIC TUBE SYSTEM.
PARIS CIRCUIT SYSTEM.]
=The Paris Pneumatic Telegraph.=—We will now glance at the system used
in Paris, which has some novel features. In 1865 it was decided to
establish a system, and the first experimental line, from Place de la
Bourse to the Grand Hôtel, on the Boulevard des Capuciens, was laid in
1866. Instead of using a steam-engine to drive an air-compressor or
exhaust-pump, air was compressed in tanks by displacement with water
from the city mains. In 1867 this line was extended to Rue de Grennelle
St. Germain, with an intermediate station at the Rue Boissy d’Anglais,
and another line with stations at Rue Jean Jacques Rousseau, Hôtel du
Louvre in the Rue de Rivoli, the Rue des Saints Pères, and terminating
in the central station. In 1868 the system was changed to a polygonal
or circuit system by removing the station in the Rue de Rivoli to the
Place du Théâtre Français and connecting the latter directly with the
Bourse. Other changes and extensions were made in 1870 and 1871, until
three polygons or circuits were formed, with five or six stations
in each circuit, and several outlying stations were connected by
independent tubes. In the middle of the year 1875 seventeen stations
had been connected and plans were made for twenty-one more. Instead of
maintaining an air-current around each circuit by machinery located at
one of the stations on the circuit, at least three of the five or six
stations comprised in the circuit have means of supplying compressed
air or of exhausting it, and each side of the polygon, or section of
the circuit between two stations, is operated independently of the
rest of the circuit (see Fig. 3). Carriers are sent on from station
to station around the circuit, either by compressed air from the last
station from which they were sent or by means of exhaustion at the
next station towards which they are moving. The carriers are made up
in trains of from six to ten, with a piston behind them that fits the
tube closely and forces them ahead. Each carrier is addressed by means
of a label for its destined station. Trains are despatched around the
circuits at stated times, usually at fifteen-minute intervals. As they
arrive at the various stations, carriers are taken out and others put
in, and the trains sent on their way. The carriers consist of iron
cylinders, closed at one end, with a leather case that slides over
them and closes the open end. They weigh, when filled with thirty-five
messages, twelve and one-half ounces, and they will travel about
twelve hundred miles before the leather cover is so worn that it must
be thrown away. The pistons are made of a wooden cone, covered with
iron, and having a “cup-leather” upon the rear end that fits the tube
closely. The sending and receiving apparatus consists of sections
of tube closed at one end, having a door on the side, through which
carriers are inserted or despatched. A peculiar form of fork is used
for picking them out. The air is controlled by valves opened and closed
by hand.
Several methods are used to compress and exhaust the air. The most
novel method consists in having tanks in which a partial vacuum is
produced by allowing water to flow out of them into the sewer, or in
having the air compressed by allowing water from the city mains to flow
into the tanks and displace the air. Water jets have also been used,
operating similar to a steam-injector. At some of the stations water
turbines drive the air-pumps, and at others steam-engines are used.
The tubes of the Paris system are of wrought iron, in lengths of from
fifteen to twenty feet, the joints being made with flanges and bolts.
The interior diameter is 2.559 inches with a maximum variation to 2.519
inches. The bends are made with a radius of from six to one hundred
and fifty feet. Water frequently gives trouble by accumulating in the
tubes. Traps are placed at low points to drain it off.
The speed of the trains of carriers in the Paris tubes is from fifteen
to twenty-three miles per hour, and the average time that elapses from
the receipt of a message until its delivery is from forty to forty-five
minutes.
=The Pneumatic Telegraph of other Cities.=—A system similar to the
one just described is used in Vienna. It differs some in details of
apparatus, but the carriers are despatched around circuits in trains,
stopping at each station, where some carriers are removed and others
inserted. Brussels also is not without its system of pneumatic tubes
for the transmission of telegrams.
=Pneumatic Tubes in America.=—Turning our attention now to our own
country, we cannot pass without mention some experiments of Alfred
E. Beach with pneumatic railways, made nearly thirty years ago. His
first experiment upon a large scale was made at the American Institute
Fair held in New York City in 1867. Here he had constructed a circular
wooden tube, one hundred and seven feet long and six feet in diameter.
A car that would seat ten people ran upon a track laid down inside the
tube, and was propelled by a helix fan ten feet in diameter, making
two hundred revolutions per minute. He next tried his railway upon
a practical scale, constructing an eight-foot tunnel for two hundred
feet under Broadway, starting at the corner of Warren Street. A car
was propelled by a large rotary blower located in the basement of a
building near by. The blower was kept constantly running, and the car
was sent alternately in one direction and then the other by changing
valves at the blower. Few people know that this experimental line still
exists under Broadway as Mr. Beach left it.
The most extensive use of small pneumatic tubes in this country has
been in our large retail department stores for despatching cash to
and from a centrally located cashier’s desk. Seamless brass tubes
are usually used, and, since the tubes are short, the air is either
compressed or exhausted by means of positive rotary blowers. At the
outlying stations the tubes are usually open to the atmosphere, while
at the central station simple forms of valves are used for sending and
receiving. An outgoing and a return tube are always used, and the air
is kept in constant circulation. The carriers are of metal with felt
packing rings and open on the side. These cash-carrying systems have
come into use during the past twenty-five years.
The Western Union Telegraph Company uses small tubes to transmit its
messages to a considerable extent in some of our large cities. In
1876 four lines were laid in New York City from the main office on
Broadway: two to the branch office at No. 14 Broad Street, one to 134
Pearl Street, and one to the Cotton Exchange. Since then this company
has laid a double line about two miles in length under Broadway to its
up-town office. It also uses tubes to send messages from the receiving
desks to the operating rooms within the buildings.
Many of our large hotels use pneumatic tubes to transmit messages to
the different floors and offices of the buildings, taking the place of
messenger, or bell boys, who formerly did this service.
We call especial attention to the fact that in all the systems that we
have mentioned which are in use both in this country and in Europe,
none of the tubes are larger than three inches internal diameter;
also that in all the systems, except in Paris where the carriers are
despatched in trains, the carriers are so light and move so slowly that
they can be stopped by allowing them to come in contact with some solid
object, such as a box into which the carriers drop. Very few of the
tubes are more than two miles in length, and most of them are less than
one mile. A speed of more than thirty miles per hour has seldom been
attempted, and usually it is much less than this.