The ornamental design for the furnace, as shown and described.
FIG. 1 is a perspective view of the furnace;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a plan view thereof, taken from above; and,
FIG. 7 is a plan view thereof, taken from below.
The present design relates to a furnace that has a cylindrical body which defines a central region for accommodating the wood to be burned surrounded by an upper circular flange and by a lower circular flange which define the area for storing the wood to be used for the furnace.