Writing to a friend in 1951, when The Lord of the Rings was nearly completed, J. R. R. Tolkien commented that though his book seemed to be largely about power, and the corruption of power, there were other issues more strongly in his mind: "Anyway all this stuff is mainly concerned with Fall, Mortality, and the Machine," and with power only as it relates to those three large themes. What did he mean by "the Machine"?