RELATION OF AIR TO THE HOUSE WE LIVE IN. 207 



the control of cleanliness, that ventilation can direct its power and 

 against this deterioration this power must be chiefly directed. 



Let us now consider the different causes of the motion of the air. 

 As air in motion is wind {ve?itus, Latin), ventilation is a better expres- 

 sion than " airing." Anything which disturbs the equilibrium of a 

 body of air, produces motion in it. Its immobility supposes equality 

 of temperature and specific gravity, and also of mixture in quantitv 

 and qviality. Such conditions, as you may suppose, are seldom present, 

 and absolute calmness is impossible. Different kinds of gases tend 

 to intermix in every direction, even contrary to their specific weight, 

 a process which is called diffusion ; but this kind of motion is not in 

 question when we s^Deak of ventilation. Ventilation means the setting 

 in motion of masses of air by mechanical pressure and the dislodging 

 of whole bodies of air similarly composed, which, for that reason, are 

 not subject to diffusion. 



We produce ventilation by disturbing the equilibrium of the air 

 in two ways : 1. By producing differences of temperature between 

 two neighboring bodies of air, which are accessible to each other ; 



2. By mechanical pressure on or driving off the air in a certain 

 direction. We cause the same motion in either way, but the first we 

 call draught, the second wind / we call forth a draught by a chimney, 

 we produce a wind by a fan, a fan-wheel, etc. 



These two factors of change of the air are continually active in 

 our houses, but to a very different extent at different times. Our 

 houses stand in the open air, which is never quite calm ; even if it 

 appears so, there is still some imperceptible motion, some wind dis- 

 posable for ventilation. Then our houses are either colder or warmer 

 than the surrounding air. They act just like large chimneys. If they 

 are colder, the air which comes in contact with them gets colder, and 

 a downward air-current is produced ; if they are warmer, the air gets 

 warmer, and an ascending current is established. 



It is evident that the intensity of the change must also depend on 

 the way in which the house is shut up, on the size and number of its 

 apertures, and the porosity of the materials it is built with. 



For this reason a certain amount of ventilation is always taking 

 place wthout any special arrangement for it, but its strength depends 

 1. On the amount of difference of temperature between outside and 

 inside ; 2. On the strength of the wind or air-motion in the open ; 



3. On the size of the apertures which are open to the change of the 

 air. We may call the first two the air-motors, the last the air-media- 

 tor or janitor, door-keeper ; to a certain degree, they can take each 

 other's place. If there is not enough difference of temperature, as, 

 for instance, in summer, the wind can act ; if both together are too 

 weak, opened doors and windows can help. In winter, when the dif- 

 ference of temperature between the in- and out-door air is consider- 

 able, small openings allow the passage of as much air as large open- 



