The answer is "with Morse code".
Morse code is a technique for transmitting content data as a progression of on-off tones, lights, or snaps that can be straightforwardly comprehended by a learned audience or onlooker without special gear. It is named for Samuel F. B. Morse. Every Morse code sign speaks to either a content character (letter or numeral) or a prosign and is spoken to by a distinct sequence of dabs and dashes.