The Jefferson Cipher was invented by a certain Mr. Thomas Jefferson, it is simple in operation and yet is still reasonably secure by todays standards (about the same strength as Vigenere - so don't trust your life to it!).
It was not adopted by the US when it was invented through some historical quirk of fate - even though it would undoubtably have withstood any contemporary cryptanalytical attack.
Instead of being placed into use it was filed in Jefferson's papers until 1922 when it was rediscovered. By coincidence, in that year the US army started using an almost identical system which was invented independently.
But what was the Jefferson wheel cipher? Simply imagine a cylinder of wood, about 15cm long and 4cm across, bore out the centre to allow a spindle to be inserted. Then slice the cylinder into slices about 5mm across.
The surface of each slice is divided into 26 sections, and one letter is assigned randomly to each section.
The slices are placed onto the spindle, and you are now ready to encode. Of course, the person receiving the message must have a similar cylinder whose wheels are arranged in exactly the same way. In the picture, you can see that the wheels are arranged to spell out the name of the cipher.
When in use the wheels are turned so that a fragment of the message appears along one side of the cylinder, the cylinder is then turned and another line is copied out at random.
This is the ciphertext.
This is repeated for each message fragment until the entire message is coded.
The decoder uses their cylinder to enter the ciphertext, and then turns the cylinder examining each row until the plaintext is seen.