Lewis Cass
Lewis Cass was the Secretary of State under President James Buchanan during the time that Townsend Harris wrote Letter No. 40. Cass was a democrat (Harris was also a democrat)(1) that served in the senate 1845-1848 and1849-1857. Cass was the civil Governor of Michigan Territory 1813-1831 and appointed Secretary of War by President Andrew Jackson 1831-1836 (2). President Buchanan selected Cass as Secretary of State in hopes that he would bring the Democratic party together. Under Buchanan, Cass negotiated the treaty that limited British and U.S. control in Latin America, known as the Clayton-Bulwer Treaty (3). The Clayton-Bulwer Treaty occurred on April 19, 1850 (4); this is before the treaty that Harris signs in Japan. This reveals that Cass had already had first hand experience with foreign affairs and treaties prior to corresponding with Harris about the treaty in Japan.
1. Finding Aid for the Townsend Harris Papers, 1855-1892 http://www.oac.cdlib.org/findaid/ark:/13030/tf0v19n715/
2. Cass, Lewis (1782-1866) http://bioguide.congress.gov/scripts/biodisplay.pl?index=c000233
3. Biographies of the Secretaries of State: Lewis Cass https://history.state.gov/departmenthistory/people/cass-lewis
4. British Diplomacy and the Clayton-Bulwer Treaty, 1850-60 by Richard W. Van Alstyne
1. Finding Aid for the Townsend Harris Papers, 1855-1892 http://www.oac.cdlib.org/findaid/ark:/13030/tf0v19n715/
2. Cass, Lewis (1782-1866) http://bioguide.congress.gov/scripts/biodisplay.pl?index=c000233
3. Biographies of the Secretaries of State: Lewis Cass https://history.state.gov/departmenthistory/people/cass-lewis
4. British Diplomacy and the Clayton-Bulwer Treaty, 1850-60 by Richard W. Van Alstyne