08/29/2026
A prison letter. A secret message. Military intelligence hidden in plain sight. Dated April 30, 1804, this letter from Captain William Bainbridge—then imprisoned in Tripoli—appeared to report the death of a seaman, a dire need for clothing, and more than a month without word from American officials.
But on the reverse was a secret message written in lime juice. Once exposed to heat, Bainbridge’s invisible words revealed estimates of the troops needed to attack Tripoli, negotiating advice, and the identity of a potential ally within the Tripoline government.
Half of Bainbridge’s surviving prison letters contain similar invisible-ink messages—evidence that even in captivity, he was working to influence negotiations and secure freedom.
🎥 Letter from William Bainbridge to Tobias Lear with Sympathetic Ink, April 30, 1804, USS Constitution Museum Collection [2147.4a,b]