It may be well to remember in the approaching cold weather, that whatever covering is used, whether straw mats, bast mats, cloth, or wood, they should be elevated above the surface to be covered, so as to contain as much confined air as possible. Confined air is one of the worst conductors of heat; the covering will not radiate, or give out heat, till the confined air and covering are both heated above the state of the atmosphere; and the transmission of heat will take place more slowly through the confined air than anything else: thus, for very little trouble, by elevating our coverings, we surround our plants or plant-structures with a substance which is very retentive of heat, and increases the power of the covering in an immense degree. The heat has most tendency to ascend upwards, and this should be moot guarded against; but it will also escape by the sides, and to confine the air and heat completely, the plant or plant-structure must be covered all round from the external air.