MLA
Wesley, John, 1703-1791. Letter from John Wesley to "Billy" William Simpson, near Longford - April 12, 1789. April 12, 1789. Wesleyana Collection. Pitts Theology Library. https://digital.library.emory.edu/purl/7719kd51vm-cor.
APA, 6th edition
Wesley, John, 1703-1791. (April 12, 1789)[Letter from John Wesley to "Billy" William Simpson, near Longford - April 12, 1789]. Wesleyana Collection, Pitts Theology Library. https://digital.library.emory.edu/purl/7719kd51vm-cor.
Chicago
Wesley, John, 1703-1791, Letter from John Wesley to "Billy" William Simpson, near Longford - April 12, 1789, April 12, 1789, Wesleyana Collection, Pitts Theology Library. https://digital.library.emory.edu/purl/7719kd51vm-cor.