According to one of the enduring legends of the Great Chicago Fire of 1871, the mayor learned that the flames were closing in on the city jail and ordered the inmates freed to save their lives.

Now, a great-great-granddaughter of the mayor has donated a slip of paper to the Chicago Historical Society that shows the legend is true.On the faded piece of police stationery, Mayor Roswell B. Mason's scribble reads: "Release all prisoners from jail at once, keeping them in custody if possible."

Society archivists authenticated the note by comparing it with samples of Mason's handwriting. They say it is significant because it apparently was written in the midst of the catastrophe, which killed about 300 people and left 100,000 homeless.

"As far as I know, we don't have anything written during the fire," says Russell Lewis, deputy director for collections and research.

The note looks like "something that had to have been done quite hastily," says Bernard Reilly, the society's director of research and access. "The fire was so out of hand, and so many decisions had to be made so quickly."

Manly W. Mumford of Chicago, a great-great-grandson of Mason and author of a biography of the mayor, says he was unaware of the note and has never met Elizabeth Trowbridge Wild of Fairport, N.Y., who donated it.

Wild says the note was in a family scrapbook that came to her 10 or 15 years ago.

As smoke began to fill the jail in the city courthouse after midnight on Oct. 9, 1871, inmates began shouting and banging on the bars. Outside, a group of people tried to smash a hole in the side of the jail and free them.

After Mason issued his order, the less dangerous inmates were simply freed. "There was a handful of murderers who were led out under guard," Reilly says.

Shortly after, Mason and others inside the courthouse fled and the building became engulfed in flames.

Legend has it that by the time the mayor made it home, he was so haggard and covered in grime that his wife did not recognize him and refused to let him in.