212 WINDS. 



movement in the opposite direction, for instance, 



by a return current in the upper regions of the 



atmosphere. The following experiment will clearly 



illustrate this. If a door be set ajar between two 



rooms, one of which is filled with cold air, the 



other with warm ; and if now a lighted candle be 



held at the crevice, at different heights one after 



another ; you will remark that at the bottom the 



flame will be turned from the cold room into the 



warm, at top it will be driven from the warm 



towards the cold room, and at some point near the 



middle height it will burn steadily upright. Prom 



this you will learn that there are two currents, one 



above the other, and in opposite directions. If a 



stove is brought quickly lip to a strong heat in a 



cold room, and a piece of paper is burned over its 



warm surface, the smell that arises will be perceived 



near the coldest wall, perhaps at the window, before 



it reaches the middle of the chamber. If several 



thermometers are placed about in the room, they 



will show more distinctly that the heated air rises 



immediately to the ceiling, that it travels along 



this to the walls, and particularly to the windows ; 



that there it is gradually cooled, and slowly falls 



till it reaches the floor, where it is often 5° or 6° C. 



(9° or 10° F.) colder than at the ceiling. On the 



floor it flows towards the stove, and there being 



warmed rises again as before. 



Just the same process goes on in the open air, 



