The Science of Discworld II

bills because they’re using the clacks all the time these days—’
    â€˜So … we’re stealing ?’ said Ridcully.
    â€˜Well, er, yes, sir, in a way, but it’s hard to know exactly what . Lastmonth Hex worked out the semaphore company’s own codes so his messages travel as part of their internal signalling, sir. No one gets billed for that.’
    â€˜This is very disturbing news, Stibbons,’ said Ridcully sternly.
    â€˜Yes, sir,’ said Ponder, looking at his feet.
    â€˜I feel I must ask you a rather difficult and worrying question: is it likely that anyone will find out?’
    â€˜Oh, no, sir. It’s impossible to trace.’
    â€˜Impossible?’
    â€˜Yes, sir. Every week Hex sends a message to company headquarters readjusting the total of messages sent, sir. Anyway, there’s so many I don’t think anyone checks.’
    â€˜Oh? Well, that’s all right then,’ said Ridcully. ‘It never really happens, and no one can find out it’s us in any case. Can we send all our messages that way?’
    â€˜Well, technically yes, sir, but I think that might be abusing the—’
    â€˜We are academics, Stibbons,’ said the Dean. ‘And information should be allowed to flow freely.’
    â€˜Exactly,’ said the Lecturer in Recent Runes. ‘An untrammelled flow of information is essential to a progressive society. This is the age of the semaphore, after all.’
    â€˜Obviously it flows to us,’ said Ridcully.
    â€˜Oh, certainly,’ said the Dean. ‘We don’t want it flowing away from us. We’re talking about flow here, not spread .’
    â€˜You wanted a message sent?’ said Ponder, before the wizards got too deeply into this.
    â€˜And we really don’t have to pay?’ said Ridcully.
    Ponder sighed. ‘No, sir.’
    â€˜Jolly good,’ said the Archchancellor. ‘Have this one sent to the kingdom of Lancre, will you? They’ve only got one clacks tower. Got your notebook? Message begins:
    â€œ To Mistress Esmerelda Weatherwax. How are you? I am fine. An interesting problem has arisen …”

EIGHTEEN
BIT FROM IT
    A SEMAPHORE IS A SIMPLE and time-honoured example of a digital communication system. It encodes letters of the alphabet using the positions of flags, lights, or something similar. In 1795 George Murray invented a version that is close to the system currently used in Discworld: a set of six shutters that could be opened or closed, thus giving 64 different ‘codes’, more than enough for the entire alphabet, numbers 0 to 10 and some ‘special’ codes. The system was further developed but ceased to be cutting-edge technology when the electric telegraph heralded the wired age. The Discworld semaphore (or ‘clacks’) has been taken much further, with mighty trunk route towers carrying bank after bank of shutters, aided by lamps after dark, and streaming messages bi-directionally across the continent. It is a pretty accurate ‘evolution’ of the technology: if we too had failed to harness steam and electricity, we might well be using something like it …
    There is enough capacity on that system even to handle pictures – seriously. Convert the picture to a 64 × 64 grid of little squares that can be black, white or four shades of grey, and then read the grid from left to right and top to bottom like a book. It’s just a matter of information, a few clever clerks to work out some compression algorithms, and a man with a shallow box holding 4,096 wooden blocks, their six sides being, yes, black, white and four shades of grey. It’ll take them a while to reassemble the pictures, but clerks are cheap.
    Digital messages are the backbone of the Information Age, which is the name we currently give to the one we’re living in, in the beliefthat we know a lot more than anyone else, ever. Discworld is comparably proud of being in the Semaphore Age, the Age of the Clacks. But what, exactly, is information?
    When you send a message, you are normally expected to pay for it – because if you don’t, then whoever is doing the work of transmitting that message for you will object. It is this feature of messages that has got Ridcully worried, since he is wedded to the idea that academics travel free.
    Cost is one way to measure things, but it depends on complicated market forces. What, for example, if