of fresh air that passes through duct 26 creates at the level of outlet 26b of this duct a partial vacuum which creates in auxiliary canal 31 obviously comes from the interior of the room under consideration and braid 19 of hygrometer 18 contacted by this flow of air is thus in good contact with the air that fills this room consequently when several individuals occupy this room and the amount of humidity in the air which fills it increases braid 19 of hygrometer 18 becomes distended and under the action of spring 27 flap 24 is pivoted in the direction of an increase in the open section of duct 26 in the example illustrated in the drawing and like that coming from fig2 two slots 16 and two flaps 24 keyed to the same shaft 23 are provided but it is obvious that a single slot 16 or more than two slots could just as well have been provided this type of air intake opening intended to be bracketed is more specially suited to outer walls of slight thickness and particularly to windows in which slots corresponding to air intake ducts 26 are provided on the contrary when an air intake opening is intended to be used with a thick outer wall an embodiment of this opening that could be incorporated into a thick wall can be provided fig4 and 5 illustrate two variant embodiments of such air intake openings incorporated into thick outer wall in the example illustrated in fig4 air intake opening 12b is incorporated into wall 15 i e it is housed almost entirely in hole 16 provided in this wall 15 and whose perimeter it fits the main duct of this opening 12b is divided into two parallel parts 32a and.