William "Duff" Armstrong

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William "Duff" Armstrong was an 1858 defendant for the murder of James Preston Metzker in Mason County, Illinois. His father, Jack Armstrong, had been a friend of Abraham Lincoln when Lincoln was studying law in New Salem, Illinois. When Lincoln heard of the murder charge, he wrote to Jack's widow, Hannah, and volunteered his legal services pro bono. The trial was moved to Cass County and held at the courthouse at Beardstown, Illinois.

Witness Charles Allen testified that he saw Duff Armstrong strike Metzker. Under cross examination Lincoln pushed for further detail and Charles Allen testified that he was at a distance of 150 feet but could clearly see the act by the light of the full moon. Abraham Lincoln used judicial notice — a very uncommon tactic at the time — to show Allen lied on the stand, when he claimed he had witnessed the crime in the moonlight. Lincoln produced a Farmer's Almanac^{[citation needed]} to show that the moon, on that date, was at such a low angle that it could not possibly have produced enough light for the "witness" to see anything clearly. Based on this evidence, the jury acquitted Armstrong after only one ballot.

Duff Armstrong went on to join the Union forces in the Civil War but died of an illness soon after.