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“To me, it was not that difficult,” he said. “I had fun with this, and it took me longer than I should have.”

The code is called the “Vigenere cipher,” a centuries-old encryption in which letters of the alphabet are shifted a set number of places so an “a” would become a “d” — essentially, creating words with different letter combinations.

The code was used widely by Southern forces during the Civil War, according to Civil War Times Illustrated.

The source of the message was likely Maj. Gen. John G. Walker of the Texas Division, who had Smith, the donor of the bottle, under his command.

The full text of the message to Pemberton reads:

“Gen’l Pemberton:

You can expect no help from this side of the river. Let Gen’l Johnston know, if possible, when you can attack the same point on the enemy’s lines. Inform me also and I will endeavor to make a diversion. I have sent some caps (explosive devices). I subjoin a despatch from General Johnston.”

The last line, Ms. Wright said, seems to suggest a separate delivery to Pemberton would be the code to break the message.

“The date of this message clearly indicates that this person has no idea that the city is about to be surrendered,” she said.

The Johnston mention in the dispatch is Gen. Joseph E. Johnston, whose 32,000 troops were encamped south of Vicksburg and prevented from assisting Pemberton by Grant’s 35,000 Union troops. Pemberton held out hope that Johnston would eventually come to his aid.

The message was dispatched during an especially terrible time in Vicksburg. Grant was unsuccessful in defeating Pemberton’s troops on two occasions, so the Union commander instead decided to encircle the city and block the flow of supplies or support.

Many in the city resorted to eating cats, dogs and leather. Soup was made from wallpaper paste.

After a six-week siege, Pemberton relented. Vicksburg, so scarred by the experience, refused to celebrate July 4 for the next 80 years.

So what about the bullet in the bottom of the bottle?

Ms. Wright suspects the messenger was instructed to toss the bottle into the river if Union troops intercepted his passage. The weight of the bullet would have carried the corked bottle to the bottom, she said.

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