Just an odd note: I've been reading Eddison's The Worm Ourobouros, first published in the early '20's.  Eddison uses the term "middle-earth" (on at least one occasion) as a synonym for "the lands, the dry land, the inhabitable part of the globe" -- i.e. precisely in that sense in which Tolkien uses the term. 
     A few quick checks in the dictionaries (I haven't had a chance to check the big OED however) revealed under the lemma "middle earth" only obsolete usages, esp. in the sense "region between Heaven and Hell" -- not the way in which either Tolkien or Eddison uses the term. 
     Has anyone noticed any other pre-Tolkien usages of "Middle-earth" (apart from translations of Anglo-Saxon or Old Norse texts where any translator might just have set "Middle-earth" to translate Middangeard or Mišgaršr)?
     What interested me in Eddison's usage was that he too used the term in its primary meaning; the meaning of "region between Heaven and Hell" is secondary and postdates the Christianisation of Scandinavia and England.  In other words, Eddison understood the term's older meaning and opted to use it in that way (just like Tolkien a little later).