INTERESTING FACT. A reference having been made by Mr. Leigh and Mr. Mercer to the 54th number of the Federalist, Mr. Madison yesterday, on a question being put by Mr. Leigh to Mr. Mercer, "who was the author of that paper?" rose and said, that although he was not desirous of saying any thing on the subject, yet, under these circumstances, he thought himself justified in stating that neither Mr. Hamilton nor Mr. Jay had written that number. In the various editions of the Federalist the authorship of this paper has been sometimes assigned to Mr. Jay, and at other times to Mr. Hamilton. Mr. Mercer had attributed it to Mr. Hamilton.