fabric around the mouthpiece with respect to overall size of the device it is thought that any dimension suggested by the representation in fig4 would be satisfactory so long as adequate volume and cross sectional area for the internal cavity is provided and the casing is not so large as to become unwieldy an exit port having a cross sectional area of between 2 and 6 square inches is foreseen depending upon the efficiency of the metallic mesh chosen formed within this outer casing 11 is a second inner casing 12 which forms a flared tube like cavity 13 characterized by a continuously increasing cross sectional area from the region around the mouthpiece to the region of the ports 14 as can be seen in fig2 this internal cavity 13 is intended to rest lightly on the face of the wearer along most of its length separated from the skin by the fabric liner 20 the purpose of this orientation is twofold by directly covering the lower face it will tend to minimize heat loss from the oral cavity of the wearer which in severely cold weather or high wind conditions can be extreme it will also maximize the accumulation and retardation of body heat by the metallic mesh within the internal cavity discussed below by allowing the oral cavity to act as an additional source of heat while even relative dimensions are not.