Feedback can either cause a change in the system or keep it stable. Melting glaciers are an example of positive feedback. Why is this considered positive feedback?

Missouri was important to the South that it be admitted to the Union as a slave state because it could tip the balance between pro-slavery and anti-slavery states in the Union.  This had become a very sensitive issue.  To solve this dilemma, it came up with the Missouri Compromise of 1820.  Under this compromise, Congress was allowed to admit Missouri as a state but it also admitted the Main to the Union as well.  This kept the balance between the states and maintained the peace.