On October 8, 1816, a special election was held in Pennsylvania's 9th district, the second special election in that district in the 14th Congress. The reason for the special election was Thomas Burnside (DR)'s resignation to accept a judicial appointment in April of that year. Burnside himself had been elected to the seat in the previous special election.
Election results
Maclay took his seat December 3, 1816, at the start of the 2nd session of the 14th Congress
References
Pennsylvania's 9th congressional district special election, 1816 Wikipedia(Text) CC BY-SA