<Tr> <Td> 7 </Td> <Td> </Td> <Td> Morrison Waite (1816--1888) </Td> <Td> January 21, 1874 (63--0) </Td> <Td> 000000001874 - 03 - 04 - 0000 March 4, 1874--March 23, 1888 (Died) </Td> <Td> 7003513300000000000 ♠ 14 years, 19 days </Td> <Td> Ulysses S. Grant </Td> <Td> Ohio State Senator (1849--1850) Presiding officer, Ohio constitutional convention (1873) </Td> </Tr> <Tr> <Td> 8 </Td> <Td> </Td> <Td> Melville Fuller (1833--1910) </Td> <Td> July 20, 1888 (41--20) </Td> <Td> 000000001888 - 10 - 08 - 0000 October 8, 1888--July 4, 1910 (Died) </Td> <Td> 7003793800000000000 ♠ 21 years, 269 days </Td> <Td> Grover Cleveland </Td> <Td> President, Illinois State Bar Association (1886) Illinois State Representative (1863--1865) </Td> </Tr> <Tr> <Td> 9 </Td> <Td> </Td> <Td> Edward Douglass White (1845--1921) </Td> <Td> December 12, 1910 (Acclamation) </Td> <Td> 000000001910 - 12 - 19 - 0000 December 19, 1910--May 19, 1921 (Died) </Td> <Td> 7003380400000000000 ♠ 10 years, 151 days </Td> <Td> William Howard Taft </Td> <Td> Associate Justice of the Supreme Court (1894--1910) </Td> </Tr> <Tr> <Td> 10 </Td> <Td> </Td> <Td> William Howard Taft (1857--1930) </Td> <Td> June 30, 1921 (Acclamation) </Td> <Td> 000000001921 - 07 - 11 - 0000 July 11, 1921--February 3, 1930 (Retired) </Td> <Td> 7003312900000000000 ♠ 8 years, 207 days </Td> <Td> Warren G. Harding </Td> <Td> 27th President of the United States (1909--1913) </Td> </Tr>

What is the main duty of the supreme court justices