of the great tubes of the Britannia Bridge. As already stated, the tubes
were built on the shore, and were floated to the towers. This was done
by introducing beneath the tubes a number of pontoons, provided with
valves in the bottom, so as to admit the water to regulate the height of
the tube according to the tide. The great tubes were so skilfully guided
into their position that they appeared to spectators to be handled with
as much ease as small boats. The mode in which they were raised by the
hydraulic presses wall be understood from Fig. 170, where A is one of
the presses and C the tube, supported by the chains, B. The tubes were
suspended in this manner at each end, and as the great tubes weighed
1,800 tons, each press had, therefore, to lift half this weight, or 900
tons. The ram or plunger of the pump was 1 ft. 8 in. in diameter, and
the cylinder in which it worked was 11 in. thick. Two steam engines,
each 40 horse-power, were used to force the water into the cylinders.
These cylinders were themselves remarkable castings, for each contained
no less than 22 tons of iron. Notwithstanding the great thickness of the
metal, an unfortunate accident occurred while the plungers were making
their fourth ascent, for the bottom of one of the cylinders gave way—a
piece of iron weighing nearly a ton and a half having been forced out,
which, after killing a man who was ascending a rope ladder to the press,
fell on the top of the tube 80 ft. below, and made in it a deep
indentation. The accident occasioned a considerable delay in the
progress of the work, for a new cylinder had to be cast and fitted. Such
an accident would assuredly have caused the destruction of the tube
itself but for the foresight and prudence of the engineer in placing
beneath the ends of the vast tube as it ascended slabs of wood 1 in.
thick, so that it was impossible for the tube to fall more than 1 in. It
must be stated that as the tube was lifted each step, the masonry was
built up from below, and then as the next lift proceeded inch by inch, a
slab of wood was placed under the ends. Although by the giving way of
the cylinder of the hydraulic press the end of the tube fell through no
greater space than 1 in., the momentum was such that beams calculated to
bear enormous weights were broken. At the time of the accident the
pressure in the cylinder did not exceed that which it was calculated to
bear or that which is frequently applied in hydraulic presses for other
purposes. Some scientific observers attributed the failure of the
cylinder to the oscillating of the tube. It had been found when the
similar tubes of the bridge over the Conway were being raised, that when
the engines at each end made their strokes simultaneously, a dangerous
undulation was set up in the tube, and it was therefore necessary to
cause the strokes of the engines to take place alternately. The chains
by which the tubes were suspended were made of flat bars 7 in. wide and
about 1 in. thick, being rolled in one piece, with expanded portions
about the “eye,” through which the connecting-bolts pass. The links of
the chain consisted of nine and eight of these bars alternately—the bars
of the eight-fold links being made a little thicker than those of the
nine-fold, so as to have the same aggregate strength. The mode in which
the hydraulic presses were made to raise the tubes is very clearly
described by Sir William Fairbairn in his interesting work on the Conway
and Britannia Bridges, and his account of the mode of raising the tubes
is here given in his own words, but with letters referring to Fig. 171:
“Another great difficulty was to be overcome, and it was one which
presented itself to my mind with great force, viz., in what manner the
enormous weight of the tube was to be kept suspended when lifted to the
height of 6 ft., the proposed travel of the pump, whilst the ram was
lowered and again attached for the purpose of making another lift. Much
time was occupied in scheming means for accomplishing this object, and
after examining several projects, more or less satisfactory, it at last
occurred to me that, by a particular formation of the links (of the
chain by which the tubes were to be suspended) we might make the chains
themselves support the tube. I proposed that the lower part of the top
of each link, immediately below the eye, should be formed with square
shoulders cut at right angles to the body of the link (Fig. 172). When
the several links forming the chain E were put together, these shoulders
formed a bearing surface, or “hold,” for the crosshead B attached to the
top of the ram A of the hydraulic pump. But the upper part of this
crosshead, C C, was movable, or formed of clips, which fitted the
shoulders of the chain, and were worked by means of right- and
left-handed screws, and could be made either to clip the chain
immediately under the shoulders when the ram of the pump was down and a
lift about to be made, or be withdrawn at pleasure. Attached to the
large girders F were a corresponding set of clips, D D, which were so
placed and adjusted as to height that when the ram of the pump was at
the top there was distance between the two sets of clips equal to twice
the length of the travel of the pump, or the length of the two sets of
the links of the chain. To render the action of the apparatus more
clear, suppose the tube resting on the shelf of masonry in the position
that it was left in after the operation of floating was completed, and
the chains attached, and everything ready for the first lift, the ram of
the pump being necessarily down. The upper set of clips attached to the
crosshead are forced under the shoulders of the links, and the lower set
of clips attached to the frames resting upon the girders are drawn back,
so as to be quite clear of the chain; the pumps are put into action
simultaneously at both ends of the tube, and the whole mass is slowly
raised until it has reached a height of 6 ft. from its original
resting-place. The clips attached to the crosshead, B, have so far been
sustaining the weight, but it will be observed that by the time the pump
has ascended to its full travel, the square shoulders of another set of
links have come opposite to the lower clips on the girders, D, and these
clips are advanced under the shoulders of the links, and the rams being
allowed to descend a little, they in their turn sustain the load and
relieve the pumps. The upper clips being withdrawn, the rams are allowed
to descend, and after another attachment, a further lift of 6 ft. is
accomplished; and thus, by a series of lifts, any height may be
attained. The fitness of this apparatus for its work was admirable, and
the action of the presses was, as Mr. Stephenson termed it, delightful.”
[Illustration:
FIG. 173.—_Apparatus to prove Transmission of Pressure in all
directions._
]
[Illustration:
FIG. 174.—_Pneumatic Tubes and Carriages._
]
PNEUMATIC DISPATCH.
When the use of the electric telegraph became general, it was found
necessary to establish in all large towns branch stations, from which
messages were conveyed to the central station, or to which they were
sent, either by messengers who carried the written despatch, or by
telegraphing between the central and branch stations. The latter had the
disadvantages of rendering the original message liable to an additional
chance of incorrect transmission, and when an unusually great number of
despatches had to be sent to or from a particular branch station, there
was necessarily great delay in the forwarding of them. The plan of
sending the written messages between the central stations by bearers was
unsatisfactory on account of the time occupied. These inconveniences led
to the invention of a system for propelling, by the pressure of air, the
papers upon which the messages were written through tubes connecting the
stations. This was first carried into practice by the Electric and
International Telegraph Company, who, in this way, connected their
central station in London with their City branch stations. The apparatus
was designed and erected by Mr. L. Clark and Mr. Varley in 1854. The
first tube laid down was from Lothbury and the Stock Exchange—a distance
of 220 yards. This tube had an inside diameter of only 1½ in.; but a
larger tube, having a diameter of 2¼ in. was, some years afterwards,
laid between Telegraph Street and Mincing Lane—a distance of 1,340
yards—and was used successfully. In these tubes the carriers were pushed
forward by the pressure of the atmosphere, a vacuum having been produced
in front by pumping out the air. The plan of propelling the carrier by
compressing the air behind it was also tried with good results, and, in
fact, with a gain of speed; for, while a carrier occupied 60 or 70
seconds in passing from Telegraph Street to Mincing Lane when drawn by a
vacuum, it accomplished its journey in 50 or 55 seconds when it was shot
forwards by compressed air, the difference in pressure before and behind
it being the same in each case. A great deal of trouble was occasioned
when the vacuum system was used, by water being drawn in at the joints
of the pipes. This water sometimes accumulated to such a degree,
especially after wet weather, that it completely overcame the power of
the vacuum to draw the air through it, by lodging in the vertical
portions of the tube, where they passed to the upper floors of the
central station. This was remedied by improving the construction of the
joints, and by arranging a syphon for drawing off any water which might
be present. The best construction of the carrier was another matter
which required some experience to discover. It was found that
gutta-percha, or papier maché covered with felt, was the most efficient
material. The tubes found by Mr. Varley to give the best results were
formed of lead covered externally with iron pipes. The joints were made
perfectly smooth in the inside by means of a heated steel mandrel, on
which they were formed, so that the tube was of one perfectly uniform
bore throughout. An ingenious arrangement was also adopted by which the
air itself was made to do the work of opening and closing the valves,
and even that of removing the carrier from the tube: when, by a
telegraphic bell, rung from the distant station, it was announced that a
carrier was dispatched, the attendant at the receiving station had only
to touch for a second a knob marked “receive,” which put the tube in
communication with the vacuum, in which condition it remained until the
arrival of the carrier, which, by striking against a pad of
india-rubber, released the detent, and thus cut off the vacuum. The
carrier then fell out of the receiver and dropped into a box placed to
catch it. When a carrier was sent, it was placed in the tube, and a
button marked “send” was touched, by which a communication was opened
with a vessel of compressed air and the end of the tube behind the
carrier was immediately closed by a slide, the movements being all
performed by the air itself. On the arrival of the carrier, the boy at
the receiving station rang an electric bell to signal its reception; and
the sender then touched another knob marked “cut off,” which caused the
supply of compressed air to be cut off, and the slide to be withdrawn
from the end of the tube, which was then ready either to receive or send
carriers. By this arrangement there was no waste of power, for the
reservoirs of compressed air or of vacuum were only drawn upon when the
work was actually required to be done.
The tubes laid down by the Telegraph Company are still in active
operation; but at the new Central Telegraph Station the automatic valves
of Messrs. Clark and Varley appear to be dispensed with, and the
attendants perform the work of closing the tube, shutting off the
compressed air, &c., by a few simple movements.
In December, 1869, Messrs. Siemens were commissioned by the
Postmaster-General to lay tubes on their system from the General Post
Office to the Central Telegraph Station; and the work having been
accomplished in February, 1870, and proving perfectly satisfactory after
six weeks’ trial, it was decided to connect in the same manner Fleet
Street and the West Strand office at Charing Cross with the Central
Station. The system proposed by the Messrs. Siemens consisted in forming
a circuit of tubes, through which the carriers might be continually
passing in one direction. The diagram, Fig. 175, will give an idea of
the manner in which it was designed to arrange the tubes between the
Central Telegraph Station and Charing Cross. The arrows indicate the
direction in which the air rushes through the tubes; A is the piston in
the cylinder, and valves are so arranged as to pump air out of the
chamber V, and compress it into the chamber P. This plan has been
departed from, so far as regards the Charing Cross Station, for want of
space there prevented the tube being curved with a radius large enough
to convey the carriers without their being liable to stick, and
consequently, these are not carried round in the tube. The passage of
carriers being stopped here, there are, in point of fact, two tubes: an
“up” tube and a “down” tube. But these are connected by a sharp bend, so
that though the tube is continuous as regards the air current, it is
interrupted as regards the circulation of the carriers. The tubes are of
iron, 3 in. internal diameter, made in lengths of about 19 ft.; and for
the turns and bends, pieces are curved with a radius of 12 ft. Both
lines are laid side by side in a trench at about a foot depth below the
streets. The ends of the adjacent lengths form butt joints, so that the
internal surface is interrupted as little as possible, and there is a
double collar to fasten the lengths together. Arrangements are also made
for removing from the inside of the tubes water or dirt, or matter which
may in any manner have got in.
[Illustration:
FIG. 175.—_Diagram of Tubes, &c._
]
[Illustration:
FIG. 176.—_Sending and Receiving Apparatus.—Transverse Section._
]
One special feature of Messrs. Siemens’ invention is the plan by which
the carriers are introduced into and removed from the tube at any
required station without the circulation of the air being interfered
with. The simple yet ingenious mechanism by which this is effected will
be understood from the sections shown in Figs. 176 and 177. The figures
represent the position of the apparatus when placed to receive a
carrier; A´ is the receptacle into which the carrier is shot by the air
rushing from A towards A´´. This receptacle is ᗜ-shaped, the curve of
the Ⅾ corresponding with that of the tube, and the upper flat part
admitting of a piece of plate glass being inserted, through which the
attendant may perceive when a carrier arrives. The progress of the
carrier is arrested by a perforated plate, B, which allows the air to
pass. The ends of this receptacle are fixed in two parallel plates, F
F´, which also receive the ends of the plain cylinder, having precisely
the same diameter as the tube, A. These plates are connected also by
cross-pieces, D E, the whole forming a sort of frame, which turns upon E
as a centre; and according as it is put in the position shown by the
plain line in Fig. 176, or in that indicated by the dotted lines, causes
the receiving tube or the hollow cylinder to form part of the main tube,
the cross-piece, D, serving as a handle for moving the apparatus. It
should be remarked that the plates are made to fit the space cut out of
the main tube with great nicety, otherwise much loss of power would
result from leakage. When the hollow cylinder is in a line with the main
tube, it is plain that the carrier will not be stopped, as the tube is
then continuous and uninterrupted. In this hollow cylinder also the
carrier to be sent is deposited after the rocking frame has been placed
on it, Fig. 177; then, on drawing the handle, the hollow cylinder is
brought into the circuit, and the carrier at once shoots off. To stop a
carrier, the receiving-tube is put in by another movement of the handle,
and when the carrier arrives, it is removed by bringing the open
cylinder, or _through tube_, into the circuit, and thus making the
receiver ready for having the carrier pushed out of it by a rod which is
made to slide out by moving a handle. In order to avoid the obstruction
to the movement of the air which would be caused by the carrier while in
the receiving-tube, a pipe, G, is provided, through which the air
chiefly passes when the perforations of the plate, B, are closed by the
presence of a carrier. In this pipe at H is a throttle-valve, which is
opened by tappets, K, on the rocking frames when the receiver is in
circuit, and again closed when the open tube is substituted. The current
thus suffers no interruption by the action of the apparatus.
[Illustration:
FIG. 177.—_Receiving Apparatus.—Longitudinal Section._
]
The carriers are small cylinders of gutta-percha, or papier maché,
closed at one end, and provided with a lid at the other. They are
covered with felt or leather, and at the front they are furnished with a
thick disc of drugget or leather, like the leathers of a common
water-pump, but fitting quite loosely in the tube. Such a carrier, being
placed in the tube at the Central Station, Fig. 175, will be carried by
the current in the direction of the arrows to the Charing Cross Station,
where its progress will be interrupted; but according to the original
plan it would continue its journey until it again reached the Central
Station, where it would be intercepted by the diaphragm, Fig. 175. But
the carrier is stopped, if at any station the receiving-tube is placed
in circuit, and this is done when an electric signal indicates to the
station that a carrier intended for it has been dispatched. The tubes
are worked on the “block system,” that is, each section is known to be
clear before a carrier is allowed to enter it, and a bell is provided,
which is struck by a little lever, moved by each carrier in its passage
through, so that the attendant at each station knows when a carrier has
shot along the “through tube” of the station. This mode of working the
tubes renders the liability to accidents much less, but their carrying
power might be increased by dispatching carriers at regular and very
short intervals of time, when the limit would be only in the ability of
the attendants to receive a carrier and open the circuit in sufficient
time to allow the next following one to proceed without stoppage. The
length of the lines of tube laid down on this system, with the times
required for the carriers to traverse them, are stated below, the
pressure and the vacuum being respectively equal to the absolute
pressures of 22 lbs. and 5½ lbs. on each square inch of the reservoirs
during the experiments:
┌────────────────────────────────────────┬──────┬──────┐
│ │Yards.│M. S. │
│Telegraph Station to General Post Office│ 852│ 1 54│
│General Post Office to Temple Bar │ 1,206│ 2 28│
│Temple Bar to General Post Office │ 1,206│ 2 10│
│General Post Office to Telegraph Station│ 852│ 1 13│
│ │ —————│ —————│
│ │ 4,116│ 7 45│
└────────────────────────────────────────┴──────┴──────┘
When the air was not compressed, but the vacuum only was used, the air
being allowed to enter the other end of the tube at the ordinary
atmospheric pressure, the time required for the carrier to traverse the
circuit was 10 minutes 23 seconds. In this case the vacuum was
maintained, so that the air was constantly in movement; but when the
experiment was tried by allowing the air in the tube to become
stationary, placing a carrier at one end, and then opening communication
with the vacuum reservoir at the other, the carrier required 13½ minutes
to complete the journey. This is explained by the fact of the greater
part of the air having to be exhausted from the tube before the carrier
could be set in motion.
The utility and advantage of the pneumatic system is well seen when its
powers are compared with the wires. Thus, a single carrier, which may
contain, say, twenty-seven messages, can be sent every eight minutes;
and since not more than one message per minute could be transmitted by
telegraph wire, even by the smartest clerks, the real average being
about two minutes for each message, it follows that only four messages
could be sent in the time required for a single carrier to traverse the
up tube, and to do the work which could be done by the tube seven wires
and fourteen clerks would be required.
Mr. R. S. Culley, the official telegraph engineer, states as his
experience of the relative wear and tear of the carriers in these iron
tubes and in the smooth lead tubes, that it had been found necessary to
renew the felt covering of eighty-two dozen of the carriers used for
three months in the iron tubes, while in the same period only
thirty-eight dozen of those used in the lead tubes required to be
recovered. The numbers of carriers sent and received by the pneumatic
tubes on the 21st of November, 1871, between 11 a.m. and 4 p.m., were:
Iron tubes │ 135
2¼ in. lead tubes 1,170│
│1,697
1½ in. lead tubes 527│
The mileage of the carriers sent was much greater in the lead than in
the iron pipes, although the total lengths of each kind were
respectively 5,974 yards and 6,826 yards. The result is remarkable, as
showing the effect of apparently slight differences when their operation
is summed up by numerous repetitions.
The circuit at Charing Cross having been divided on account of the
difficulty mentioned above, the tubes act as separate pipes—one for “up”
traffic (_i.e._, to Central Telegraph Station), the other for “down”
(_i.e._, from the Central Station). The air, however, still accomplishes
a circuit, being exhausted at one end and compressed at the other. A
very noticeable and curious difference is found between the times
required by the carriers to perform the “up” and the “down” journeys:
An “up” carrier requires 6·5 minutes
A “down” carrier requires 12·5 minutes
————
Together 19·0 minutes
When two pipes were separated at Charing Cross so that the air no longer
circulated from one to the other, but both were left open to the
atmosphere, while the “up” pipe was worked by a vacuum only and the
“down” pipe by pressure only, the times were for
An “up” carrier 8·5 minutes
A “down” carrier 11·3 minutes
————
Together 19·8 minutes
The time, therefore, for the whole circuit was practically the
same—whether the tubes were worked by a continuous current of air or
separated, and one worked by the vacuum and the other by pressure. It
was also seen that when the tubes were connected so that the air current
was continuous, and the pump producing a vacuum at one end and a
compression at the other, the neutral point where the pressure was equal
to that of the atmosphere was not found midway between the two
extremities—that is, at Charing Cross Station—but much nearer the vacuum
end. When the tubes were disconnected, it appeared, as already shown by
the figures given above, that there was a gain of speed on the down
journey, and a loss of speed on the up journey; and as the requirements
of the traffic happened to require greater dispatch for the down
journeys, the tubes have been worked in this manner.
It has been proposed to convey letters by pneumatic dispatch between the
General and Suburban Post Offices, and the Post Office authorities have
even consulted engineers on the practicability of sending the Irish
mails from London to Holyhead by this system. It was calculated,
however, that although the scheme could be carried out, the proportion
of expense for great speeds and long distances would be enormously
increased. A speed of 130 miles per hour was considered attainable, but
the wear and tear of the carriers would be extremely great at this high
velocity, and it was considered doubtful whether this circumstance might
not operate seriously against the practical carrying out of the plan.
The prime cost would be very great, for the steam power alone which
would be requisite would amount to 390 horse-power for every four miles.
We thus see that very high velocities would introduce a new order of
difficulties in the practical working. The case as regards the velocity
with which electric signals can be sent round the world is very
different.
An amusing hoax appears to have been perpetrated by some waggish
telegraph clerk on an American gentleman at Glasgow, with regard to the
pneumatic system of sending messages; for the gentleman sent to the
“Boston Transcript” a letter, in which he relates that having sent a
telegraphic message from Glasgow to London, he received in a few minutes
a reply which indicated a mistake somewhere, and then he went to the
Glasgow telegraph office, and asked to see his message.
“The clerk said, ‘We can’t show it to you, as we have sent it to
London.’ ‘But,’ I replied, ‘you must have my original paper here. I wish
to see that.’ He again said, ‘No, we have not got it: it is in the post
office at London.’ ‘What do you mean?’ I asked. ‘Pray, let me see the
paper I left here half an hour ago.’ ‘Well,’ said he, ‘if you must see
it, we will get it back in a few minutes, but it is now in London.’ He
rang a bell, and in five minutes or so produced my message, rolled up in
pasteboard.... I inquired if I might see a message sent. ‘Oh, yes; come
round here.’ He slipped a number of messages into the pasteboard scroll,
popped it into the tube, and made a signal. I put my ear to the tube and
heard a slight rumbling noise for seventeen seconds, when a bell rang
beside me, indicating that the scroll had arrived at the General Post
Office, 400 miles off. It almost took my breath away to think of it.”
In the journal called “Engineering,” into which this curious letter was
copied, it is pointed out that to travel from London to Glasgow, a
distance of 405 miles, in seventeen seconds, the carrier must have moved
at the rate of 24 miles per second, or 5 miles a second faster than the
earth moves in its orbit, and the carrier would have in such a case
become red hot by its friction against the tube before it had travelled
a single second.