The Collaborative International Dictionary of English v.0.48:

Wellhole \Well"hole`\, n.
   [1913 Webster]
   1. (Arch.)
      (a) The open space in a floor, to accommodate a staircase.
      (b) The open space left beyond the ends of the steps of a
          staircase.
          [1913 Webster]

   2. A cavity which receives a counterbalancing weight in
      certain mechanical contrivances, and is adapted also for
      other purposes. --W. M. Buchanan.
      [1913 Webster]