The ornamental design for a furnace, as shown and described.
FIG. 1 is an upper, right, front perspective view of a furnace showing my new design
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 ornamental design which is claimed is shown in solid lines in the drawings.
The broken lines in the drawings depict unclaimed portions of the furnace. The broken lines and portions of the furnace they depict form no part of the claimed design.