The first Chappe telegraph line linked Paris to Lille: fifteen towers over 230 kilometres, each watched by telescope from the next, a message crossed in about nine minutes. The operators in the middle were never given the code book. They saw two numbers and repeated two numbers. This page is those towers. Send a word, and it leaves here as two numbers that this page cannot read back to you. The book is printed below. The numbers are all that passed.
You are at the origin office, where the book is kept. Send a word: the office looks it up, and the word travels as two numbers, page then entry. Every tower between repeats the pair.
This page is the towers, and the towers do not hold the book. It can tell you what it relayed. It cannot tell you what the numbers meant, and it will not keep the word you sent. That is the design of the line, not a fault of it.
The demonstration book below carries ninety-two words on one page. The real book ran to ninety-two pages of ninety-two entries: 8,464 words and expressions in all. A word the book cannot carry does not travel.
Each word travels as two numbers: the page, then the entry. A full book had ninety-two pages. This demonstration book has one.
Why the operators were blind: each tower kept two men, one to read the incoming arms and one to work the outgoing ones, and the man who sent a pair had to watch the next tower repeat it back correctly. Neither held the book, so the line carried what its stations could not read. In 1836 two businessmen in Bordeaux were caught using the line for Paris stock prices ahead of the public; they had bribed the tower men, and the men who relayed those numbers could not read them either.
Sources: Wikipedia, “Chappe telegraph” (fetched 4 August 2026). The page is a working model: the arms each number makes are this model’s own assignment; the real mapping of the 92 numbers onto the regulator and the two indicators is a matter of record, and this page does not reproduce it. The code book is a demonstration subset of the real 8,464-word book. What is not a model: the line’s design, the blind operators, and the prices that travelled without being read.