S Your new gas furnace may have been installed in one of two. ways, as a direct-vent (2-pipe-Fig. 2) application or as a. non-direct vent (1-pipe-Fig. 3) application. S In a direct-vent (2-pipe) application,your furnace uses air. from outside the home for combustion and ventsflue gas to the. outdoors.
For the biological concept, see Anti-predator adaptation. A defence mechanism is an unconscious psychological mechanism that reduces anxiety arising from unacceptable or potentially harmful stimuli. Sigmund Freud was one of the first proponents of this construct.
NATO refers to airborne air defence as counter-air and naval air defence as anti-aircraft warfare. Missile defence is an extension of air defence as are initiatives to adapt air defence to the task of intercepting any projectile in flight.