August 26 – John Fitch is granted a patent for the steamboat in the United States.
September 5 – An ordinance is written barring the game of baseball within 80 yards of the Meeting House in Pittsfield, Massachusetts, the first known reference to the game of baseball in North America.[3]
September 9 – Washington, D.C. is officially named after the incumbent President.
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Wood, Betty (1992). "White women, black slaves and the law in early national Georgia: The Sunbury petition of 1791". The Historical Journal. 35 (3): 611–622. doi:10.1017/S0018246X00025991.
Eid, Leroy V. (1993). "American Indian Military Leadership: St. Clair's 1791 Defeat". The Journal of Military History. 57 (1): 71–88. doi:10.2307/2944223. JSTOR2944223.
Tim H. Blessing. The Lewistown riots, 1791–1793: a micro-analytic approach. Pennsylvania History, Vol. 71, No. 3 (Summer 2004), pp. 285–321