MLA

Sewell, George, -1726, author. Observations upon Cato, a tragedy by Mr. Addison : in a letter to ***.. MDCCXIII [1713]. https://collections.library.yale.edu/catalog/30598187.

APA, 6th edition

Sewell, George, -1726, author. (MDCCXIII [1713])[Observations upon Cato, a tragedy by Mr. Addison : in a letter to ***.]. https://collections.library.yale.edu/catalog/30598187.