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 (ventus, Latin), ventilation is a better expression 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 quantity and quality. 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 speak 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 disposable 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-mediator 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 difference of temperature between the in-and out-door air is considerable, small openings allow the passage of as much air as large open-
