Thanks to Project Gutenberg, I am reading The Count of Monte Cristo. It's a good television book. Then I got to this bit:
"'And how did this despatch reach you?' inquired the king. The minister bowed his head, and while a deep color overspread his cheeks, he stammered out, --
'By the telegraph, sire.' -- Louis XVIII advanced a step, and folded his arms over his chest as Napoleon would have done."
Wha? This book begins in 1814. Though Dumas wrote it after Samuel Morse invented the telegraph in 1842, it was only three years afterward, which was only one year after the first complete line was laid between Baltimore and D.C.
12 March: Aha--thanks to Beth, the explanation I should have looked for. Dumas wasn't being prescient but describing Chappe's telegraph, which appears to have been a mechanical semaphore. Or I should say, the semaphore is a descendant of Chappe's telegraph, which surprises me. I would have guessed that the semaphore had been invented earlier, to complement or assist Europe's naval supremacy. But then, the longitude clock wasn't invented until well into the eighteenth century, and the semaphore is much less critical than that.