660 THE POPULAR SCIENCE MONTHLY. 



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 move- 

 ments 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 sub- 

 stances. If, for instance, an escape of gas were to take place in a re- 

 mote 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 3^ 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 con- 

 clusions from other observations ; but you may easily convince your- 

 self 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 instru- 

 ment 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 ajbout the height and width of a human body; let us 

 say it measures about 6 feet by 1|, 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 venti- 

 lation 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 con- 

 sidered necessary in the ventilating arrangements of hospitals, etc. 

 This quantity, which appears large, is after all only about -^ of the 

 quantity of air which comes in .contact with a person in the open at 

 the above stated average velocity of the air. 



