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The history of optical telegraphs (Language of Coins 5/9)

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    The signal fire is no doubt one of the oldest technologies
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    for transmitting information –
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    perhaps dating back to the first controlled use of fire.
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    It allows one person to influence another's belief state –
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    across a distance.
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    Because with the ability to notice
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    either the presence or absence of something,
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    we are able to switch between one of two belief states.
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    One difference. Two states.
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    And ff we look back in history,
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    we find that this was of great importance
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    to military powers,
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    which all rely on effective communications.
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    And a great place to begin
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    is with the Greek myth of Cadmus –
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    a Phoenician prince who introduced
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    the 'phonetic' letters to Greece.
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    The Greek alphabet –
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    borrowed from the Phoenician letters –
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    along with light, and cheap, papyrus –
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    effected the transfer of power
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    from the priestly to the military class.
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    And Greek military history provides clear evidence
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    of the first advancements in communication,
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    stemming from the use of signal torches.
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    Polybius was a Greek historian born in 200 BC.
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    He wrote 'The Histories,' which is
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    a treasure trove of detail related to
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    the communication technologies of the time.
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    He writes: "The power of acting at the right time
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    contributes very much to the success of enterprises.
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    And fire signals are the most efficient of all devices
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    which aid us to do this."
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    However, the limitation of a signal fire was clear to him.
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    He writes:
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    "It was possible for those who had agreed on this
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    to convey information that, say, a fleet had arrived.
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    But when it came to some citizens
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    having been guilty of treachery,
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    or a massacre having taken place in town –
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    things that often happen, but cannot all be foreseen –
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    all such matters defied communication by fire signal."
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    A fire signal is great when
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    the space of possible messages is small –
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    such as enemy has arrived or not arrived.
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    However, when the message space – which is
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    the total number of possible messages – grows,
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    there was a need to communicate many differences.
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    And in The Histories, Polybius describes a technology
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    developed by Aeneas Tacticus –
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    one of the earliest Greek writers on the art of war –
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    from the 4th century BC.
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    And his technology was described as follows:
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    "Those who are about to communicate
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    urgent news to each other by fire signal
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    should procure two vessels
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    of exactly the same width and depth.
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    And through the middle should pass a rod,
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    graduated into equal sections –
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    each clearly marked off from the next,
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    denoted with a Greek letter."
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    Each letter would correspond to
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    a single message in a look-up table which contain
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    the most common events that occur in war.
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    To communicate, they would proceed as follows:
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    First, the sender would raise his torch
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    to signal he had a message.
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    The receiver would then raise his torch,
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    signaling he was ready to receive it.
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    Then, the sender would lower his torch,
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    and they would both begin to drain their vessels
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    from a bored hole of equal size at the bottom.
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    Now, when the event is reached,
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    the sender raises his torch
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    to signal that they should both stop the flow of water.
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    This results in equal water levels,
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    denoting a single shared message.
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    This ingenious method
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    used differences in time to signal messages.
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    However, its expressive capabilitiy was limited,
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    mainly due to its speed.
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    Polybius then writes of a newer method –
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    originally devised by Democritus –
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    which he claims was "perfected by myself,
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    and quite definite and capable of dispatching –
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    with accuracy –
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    every kind of urgent message."
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    His method – now known as the 'Polybius Square' –
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    works as follows:
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    Two people, seperated by a distance,
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    each have 10 torches –
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    separated into two groups of five.
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    To begin, the sender raises a torch
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    and waits for the receiver to respond.
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    Then, the sender lights a certain number
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    from each group of torches – and raises them.
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    The receiver then counts
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    the number of torches lit in the first group.
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    This number defines the row position
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    in an alphabetic grid they share.
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    And the second group of torches
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    signifies the column position in this grid.
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    The intersection of the row and column number
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    defines the letter sent.
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    Realize, this method can be thought of
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    as the exchange of two symbols.
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    Each group of five torches is a symbol,
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    which was limited to five differences –
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    from one to five torches.
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    Together, these two symbols multiply
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    to give 5 x 5 = 25 differences –
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    not 5 + 5.
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    This multiplication demonstrates
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    an important combinatorial understanding in our story.
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    It was explained clearly in a 6th-century-BC
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    Indian medical text, attributed to Sushruta –
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    an ancient Indian sage – as follows:
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    "Given 6 different spices,
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    how many possible different tastes can you make?"
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    Well, the process of making a mixture
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    can be broken down into in six questions:
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    Do you add A? Yes or no?
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    Do you add B?
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    C?
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    D?
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    E?
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    and F?
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    Realize, this multiplies into
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    a tree of possible answer sequences –
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    2 x 2 x 2 x 2 x 2 x 2 = 64 ...
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    64 different sequences of answers
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    are therefore possible.
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    Realize that given n yes-or-no questions,
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    there are 2 to the power of n possible answer sequences.
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    Now in 1605, Francis Bacon clearly explained
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    how this idea could allow one to send
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    all letters of the alphabet,
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    using only a single difference.
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    [Regarding] his 'bilateral cipher,' Bacon wrote, famously:
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    "The transposition of two letters by five placings
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    will be sufficient for 32 differences.
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    For by this art, a way is opened whereby a man
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    may express and signify the intentions of his mind –
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    at any distance of place – with objects which are capable
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    of a two-fold difference only."
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    This simple idea of using a single difference
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    to communicate [all of the letters of] the alphabet
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    really took flight in the 17th century,
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    due to the invention of the telescope
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    by Lippershey, in 1608, and Galileo, in 1609.
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    Because quickly, the maginification power of the human eye
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    jumped from 3, to 8, to 33 times – and beyond.
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    So the observation of a single difference
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    could be made at a much greater distance.
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    Robert Hooke, an English polymath interested in
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    improving the capability of human vision, using lenses,
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    ignited progress when he told the Royal Society, in 1684,
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    that suddenly, "with a little practice,
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    the same character may be seen at Paris,
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    within a minute after it hath been exposed at London."
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    This was followed by a flood of inventions
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    to pass differences more effectively
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    across greater distances.
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    One technology, from 1795, perfectly demonstrates
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    the use of a single difference to communicate all things.
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    Lord George Murray's 'shutter telegraph'
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    was Britain's reaction to the Bonapartist threat to England.
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    It was composed of six rotating shutters,
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    which could be oriented as either 'open' or 'closed.'
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    Here, each shutter can be thought of as a single difference.
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    With six shutters, we have six questions: open or closed –
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    providing us with 2^6, or 64, differences –
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    enough for all letters, digits, and more.
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    Now realize that each observation of the shutter telegraph
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    can also be thought of as the observation
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    of one of 64 different paths through a decision tree.
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    And with a telescope, it was now possible to send letters
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    at an incredible distance between beacons.
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    However, an observation in 1820
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    led to a revolutionary technology,
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    which forever changed how far these differences
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    could travel between signaling beacons.
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    This ushered in new ideas
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    which launched us into the 'Information Age.'
Τίτλος:
The history of optical telegraphs (Language of Coins 5/9)
Περιγραφή:

History of optical telegraph signals from simple display to the eighteenth century semaphore telegraph. We will follow the ideas of Polybius, Aeneas Tactics, Sushruta, Francis Bacon, Robert Hooke and others.

Sources:
Communications: An International History of the Formative Years (Burns)
Understanding Media (McLuhan)
The Mathematical Theory of Communication (Shannon)
The Histories (Polybius)
translation link: http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Polybius/home.html)

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Video Language:
English
Duration:
09:20

English subtitles

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