BIRNBAUM, a circle in the Prussian government of Posen, formerly a part of Poland. It is extended over 543 square miles, or 347,520 acres, and comprehends five cities or towns, and 150 villages, with 26,923 inhabitants. The northern part is sandy and woody, but the southern is moderately fertile. The chief place, which gives its name to the circle, stands on the river Warthe, near a lake, and contains 2010 inhabitants, of whom one fourth are of the Jewish nation.