On May 1, 1826, Alexander Thomson (J) of Pennsylvania's 13th district resigned. A special election was held to fill the resulting vacancy on October 10, 1826, the same day as the general elections to the 20th Congress.
Election results
Forward took his seat on December 4, 1826, at the start of the Second Session of Congress.
References
Pennsylvania's 13th congressional district special election, 1826 Wikipedia(Text) CC BY-SA