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