Source : Webster's Revised Unabridged Dictionary (1913)

Snifting \Snift"ing\,
   a. & n. from {Snift}.

   {Snifting valve}, a small valve opening into the atmosphere
      from the cylinder or condenser of a steam engine, to allow
      the escape of air when the piston makes a stroke; -- so
      called from the noise made by its action.