FURNACES.
There is no improved mechanism in the economy of domestic life requiring
more intelligent management than furnaces. Let us then consider some
of the principles involved.
The earth is heated by radiation from the sun. The air is not warmed
by the passage of the sun's heat through it, but by convection from
the earth, in the same way that it is warmed by the surfaces of stoves.
The lower stratum of air is warmed by the earth and by objects which
have been warmed by radiated heat from the sun. The particles of air
thus heated expand, become lighter, and rise, being replaced by the
descent of the cooler and heavier particles from above, which, on being
warmed also rise, and give place to others. Owing to this process, the
air is warmest nearest the earth, and grows cooler as height increases.
The air has a strong attraction for water, and always holds a certain
quantity as invisible vapor. The warmer the air, the more moisture it
demands, and it will draw it from all objects within reach. The air
holds water according to its temperature. Thus, at fifty-two degrees,
Fahrenheit's thermometer, it holds half the moisture it can sustain;
but at thirty-six degrees, it will hold only one eighty-sixth part.
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