[The Mouth of Sauron]

In the Return of the King, Aragorn tells Pippin that the Orthanc Stone will be "used by the King". But in later writings, Tolkien states that Aragorn decided to set up the Orthanc Stone permanantly within the Tower of Orthanc.

Presumably therefore Aragorn had mastered the Minas Tirith Stone in order that he could see more than just Denethor's hands withering in flame ? Otherwise surely he would never have set up the one working Stone in relatively isolated Isengard ?