After Jefferson won the 1800 presidential election, Madison served as Secretary of State from 1801 to 1809. In that position, he supervised the Louisiana Purchase, which doubled the size of the United States. From age 11 to 16, Madison was sent to study under Donald Robertson, a Scottish instructor who served as a tutor for a number of prominent planter families in the South. He remained at Princeton to study Hebrew and political philosophy under President John Witherspoon before returning home to Montpelier in early 1772. Madison began to study law books on his own in 1773. Madison asked a Princeton friend William Bradford, a law apprentice under Edward Shippen in Philadelphia, to send him an ordered written plan on reading law books. At the age of 22, there was no evidence that Madison, himself, made any effort to apprentice under any lawyer in Virginia.