From an article in the August 2008 issue of Smithsonian Magazine, I learned that soldiers stationed along the Great Wall in China used to communicate by sending smoke signals during the day and lighting fires at night.

Different threats were denoted by the magnitude of these signals. "An incursion of 100 men required one lighted beacon and a round of cannon fire... 5000 men merited five plumes of smoke and five cannon shots."

The cannon part must have been fairly late in history, but the smoke part is ancient. Chinese literature mentions "a rash of wolf smoke across the land", which refers to the signals created by burning wolf dung, something which produced a tall, straight column of smoke. I wonder why wolf dung burned differently than the dung of something else.

It was also Imperial policy that the grass and trees be removed, often by fire, from sixty miles on either side of the Great Wall, so that any approaching enemy would have no cover. This cleared land was used for farming, but over the years, the lack of natural vegetation has allowed the desert to creep in, destroying the land's agricultural value. This same desertification now threatens the Wall itself.


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