On December 15, 1829, a special election was held in Pennsylvania's 16th congressional district to fill a vacancy caused by the resignation of Representative-elect William Wilkins (AM) on November 9, 1829, prior to the start of the 21st Congress.
Election results
Denny took his seat on December 30, 1829.
References
Pennsylvania's 16th congressional district special election, 1829 Wikipedia(Text) CC BY-SA