such it is displaced by colder and heavier air, which in its turn gets warmer and lighter, and so on.
Each person standing in the still air of a room causes in this way an ascending current of air, just like a heated stove. A very sensitive anemometer, placed between coat and waistcoat, shows the existence of this current, which is strong enough to set the little wings of the instrument in play. The air in this room appears quite still, and yet it is in thousand-fold motion and ceaseless restlessness; but, happily, our nerves are not aware of this, just as a short-sighted person may deny the existence of some object, till his eyes get the assistance of a glass. Whoever of you would be able to feel or see all the movements of the air in this room would probably not be able to stand it. A correct idea may be formed about it by the action of smelling substances. If, for instance, an escape of gas were to take place in a remote corner of this large room, you would become aware of it almost immediately all over the room. Our nerves are happily so organized that they begin to feel the motion of the air only when it amounts to about 34 feet per second.
You may have some doubt about this ignorance of your nerves, because the proof lies not in our immediate perception, but in conclusions from other observations; but you may easily convince yourself that it is so. It is the same thing whether you move your hand at a certain rate through a still air, or whether air moves at the same rate round your hand. You will find that you do not feel anything, no resistance, no coolness, if you move your hand at less than 19 inches per second.
I take this opportunity to draw your attention at once to the average movement of air out-of-doors, a subject very imperfectly known to most people, but which you must understand well in order to have a correct idea of the real difference between being in a room and in the open air. The velocity of the air is measured by an instrument called an anemometer, a description of which you can easily get at. In our temperate climate this velocity amounts on an average to about 10 feet per second. This would make about 7 miles per hour. Imagine a frame about the height and width of a human body; let us say it measures about 6 feet by 12, or 9 square feet. If you multiply this by the average velocity of the air, you will find that in one second 90 cubic feet, in one minute 5,400 cubic feet, in one hour 324,000 cubic feet of air flow over one person in the open. I shall come back again to these numbers when we have to consider the subject of the ventilation of dwellings, but you will already understand that it is not too much if 2,100 cubic feet of new air per hour and per bed are considered necessary in the ventilating arrangements of hospitals, etc. This quantity, which appears large, is after all only about 160 of the quantity of air which comes in contact with a person in the open at the above stated average velocity of the air.