8 July 1811, Washington. “A letter lately received from bishop Madison, inclosing a statement to him from Mr. Blackburn, professor of mathematics at William and Mary College, relative to a supposed error in the calculation of the longitude of that college, transmitted by you some weeks ago,1 has made it necessary for me to explain the objectionable part of the computation, by demonstrating to the bishop, a misapplication of the rule referred to by Mr. Blackburn. The inclosed will, it is hoped, remove any remaining doubt on the subject.

“As Dr. Maskelyne2 and M. de la Lande,3 have proposed different rules to ascertain the Moon’s parallax in latitude, which do not give the same result, it is essential in practical astronomy, that both and each of those rules should be investigated; for upon the correct determination of that element, the longitude of a place, especially one far distant from another meridian, will materially depend.”