VENTILATORS. j 55

a fresh Air, it descends to the lower Part ofthe Lungs, and thereby drives out the lighterwarm foul Air: But when in crowdedRooms, we breathe an Air as hot and dampas that in the Lungs, we then only changein breathing the upper Air in the Lungs, thelower Air being equally hot and light, as thenew-breathed upper Air, and, consequently,of equal Weight, the lower Air cannot bechanged) but by continuing to grow foulerand fouler, gives that uneasy Sensation whichis called a pent Breath. Thus I have foundby Experiments, as have several otheis, thatthe fame Gallon of Air can scarcely, and withgreat Difficulty, be breathed to and fro for aMinute, by reason of its great Degree ofFoulness. It is, therefore, a Matter of greatImportance, to use Means to change the Airof such Rooms, by a constant gentle Suc-cession of fresh: But it must not be let inat full open Windows, especially in coldWeather, because it will fall down precipi-tately, and incommode the Company. Therewill be another great Convenience in thuschanging the Air of crowded Rooms, viz.that they will not, when they go out, havethe dangerous Inconvenience of a sudden

Change,