Last Sunday evening, I watched an excellent Masterpiece Contemporary movie on PBS called "Page Eight", about an aging spy uncovering a plot to possibly bring down the British government. It was suspenseful, pulled you in immediately, with a great cast of British stage and film actors, and filmed on location in London (an added plus).
Got me to thinking about espionage in the 18th Century compared with 20th Century spies and new technology. I found some very interesting stuff. During the Revolutionary War, spies were skulking around America and England in full force, transmitting information on troop movements, supplies, all types of political manuevering. Technological trends included invisible ink, secret codes and blind drops which seem quite tame, but for their day, they were state-of-the-art.
One kind of invisible ink was heat activated. The person wrote regular correspondence, and wrote again between the lines with the chemical that when heated, would appear! Pretty progressive for the times (see example below).
Spying went so far as against family members, with William Franklin, who was a Tory (royal loyalist) spying on his father Benjamin!