68 HEAT OR CALORIC. 



Here again the common impressions are erroneous. Air is com- 

 monly used to cool bodies, but it is air in motion, not air at rest. Air 

 in motion cools hot bodies rapidly, because new particles come every 

 moment into contact with the heated body. Air confined, impedes 

 the progress of heat more than any other body, because it is among 

 the very worst of conductors. 



Double windows, double walls, furred* walls, all contribute very 

 much to keep houses warm in winter and cool in summer, because 

 the parallel surfaces imprison the air between them. 



(n.) Change of temperature instantly disturbs the statical pressure 

 of the air and produces currents. A common fire, a lamp, a candle, 

 and all furnaces, are examples. 



When the fire is active, there are opposite currents in a warm room, 

 of cold air along the floor, and of warm air along the ceiling. The 

 currents divide at an open door ; hot air passes out above, and cold 

 air blows in below, as may be seen by placing the flame of a candle in 

 the door ; above, it will point outward ; below, inward, and . at an in- 

 termediate point, it will be perpendicular ; or, three candles may be 

 used at the same time, and the effects will be as stated above : the 

 hotter the room and the colder the external air, the more striking will 

 be the effect. 



(0.) The best air for respiration is usually along the floor. Peo- 

 ple falling from suffocation, in bad air, often recover on reaching the 

 floor ; a principal, although not perhaps the sole reason, is, because the 

 deadly gases and vapors, if not specifically lighter than air, are usually 

 temporarily so from their rarefaction, as they are commonly produced 

 either by respiration-)- or combustion. A life preserver used in fires, 

 is worn on the head, and a projecting flexible tube descends like an el- 

 ephant's proboscis, so that the orifice or snout touches the floor, and 

 thus the wearer breathes, it may be, tolerable air, while that which 

 surrounds his head, would, if inspired, be noxious or perhaps fatal. 



(p.) The current of a chimney and of common winds, as well as 

 monsoons, trade winds, and even hurricanes and tornados, depends on 

 the ascent of air rarefied by heat. Warm air, that is to say, lighter 

 air is forced upward by colder, or in other words, by heavier air. 



The monsoons of India are produced by the heating of the earth, 

 and consequently of the air, by the sun, during his visit to the northern 

 tropic : the colder air, from the ocean consequently rushes in to restore 



* Furred, a term applied by the builders to an interior wall in a stone or brick 

 house, laid not upon tlje solid material, but upon lath, which are nailed to perpendicu- 

 lar strips of boards or plank, and these again to billets of wood laid in the masonry ; 

 there is then a space filled with imprisoned air, 



t In bed rooms, especially in cold weather, carbonic acid gas, flowing rarefied 

 from a hot source, may afterwards become so chilled, as (o fall and prevail most near 

 the floor ; a pan of cools or even a lamp or ^ candle may in this manner, especially 

 in a small room, without an open chimney, produce a noxious atmosphere. 



