The ornamental design for a furnace, as shown and described.
FIG. 1 is an upper, right, front perspective view of a furnace.
FIG. 2 is a front view of the furnace shown in FIG. 1.
FIG. 3 is a rear view of the furnace shown in FIG. 1.
FIG. 4 is a left side view of the furnace shown in FIG. 1.
FIG. 5 is a right side view of the furnace shown in FIG. 1; and,
FIG. 6 is a top view of the furnace shown in FIG. 1.
The broken lines in the drawings form no part of the claimed design. Broken lines formed by equal length dashes show unclaimed subject matter. Broken lines formed of unequal length dashes (i.e., dash-dot) define bounds of the claimed design.