Source : Webster's Revised Unabridged Dictionary (1913)

Shrinking \Shrink"ing\,
   a. & n. from {Shrink}.

   {Shrinking head} (Founding), a body of molten metal connected
      with a mold for the purpose of supplying metal to
      compensate for the shrinkage of the casting; -- called
      also {sinking head}, and {riser}.