These big ol' things aren't used anymore because they polluted the air. At one time I guess these were pretty common in the areas where logging took place. I think they might have been used to burn up the small branches and maybe the bark that was stripped off the logs before they were shipped somewhere to a sawmill. Whatever...they are pretty impressive and big! I think this one, which is at the outdoor museum in Fort Missoula, Montana, is about forty feet in diameter.
(Click on photo to open large size view.)