The writer believes that ere long the common mode of warming by furnaces will be banished as most pernicious to health, and constant sources of discomfort and economic waste. The reasons for this demand reference to some of the principles of pneumatics.
It has been shown how the air is heated by convection, or changing contact. It is thus the atmosphere is warmed, not by the rays of the sun passing through it, but by contact with the earth and other objects which have been warmed by radiated heat from the sun. The lower stratum of air being thus warmed, becomes lighter, and ascends, giving place to the cooler and heavier air. This process continues, so that the warmest air is always nearest the earth, and grows cooler as height increases.
The air has a strong attraction for water, and always holds a certain quantity as an invisible vapor. The warmer the air the more water it demands, and will draw it from all objects it can reach. When air cools, it deposits its invisible moisture as dew. When the air has all the water it can hold, it is said to be saturated; and when it cools so as to begin to deposit moisture, it is called the dew point.
When air holds all the moisture it can sustain, its moisture is said to be at 100 per cent.; when it holds only one-half as much as its temperature demands, it is said to be at 50 per cent.; and when it holds three-fourths of what its temperature requires, it is at 75 per cent.; and when only one-fourth, it holds 25 per cent.
In summer, outdoor air rarely holds less than half its volume of water; that is, a quart of air usually holds as much as a pint of invisible vapor. In 1838, at Harvard and Yale, at 70° Fahrenheit, the air held 80 per cent. of moisture; at New Orleans it often holds 90 per cent.; at the North, in fogs, the air often holds all it can, or is saturated—that is, holding 100 per cent. Thus it appears that the hotter the air, the more water is demanded by it for invisible vapor, and this it takes from all around.
Professor Bremer, of Yale College, states that 40 per cent. of moisture is needed to make air healthful. Now furnaces receive cold air containing little invisible moisture, and by heating it a demand is created for much more. This is sucked up, as by a sponge, from walls and furniture, and especially from the lungs and capillaries of our bodies, thus causing dryness and sometimes inflammation of lips, nose, eyes, throat, and lungs. Experiments prove that while 40 per cent. of moisture is needed for health, furnace-heated air rarely has as much as 20 per cent., even when a few quarts of water are evaporated in the furnace chamber. Thus the inmates of the house breathe dryer air than is ever breathed in the hottest deserts of Sahara.
Thus, for want of proper instruction, most American housekeepers who use stoves and furnaces not only poison their families with carbonic acid and carbonic oxide, and starve them for want of oxygen, but also diminish health and comfort for want of a due supply of moisture in the air. And often when a remedy is sought, by evaporating water in the furnace, or on the stove, it is without knowing that the amount evaporated depends, not on the quantity of water in the vessel, but on the extent of evaporating surface exposed to the air. A quart of water in a wide shallow pan will give more moisture than two gallons with a small surface exposed to heat.
There is also no little wise economy in keeping a proper supply of moisture in the air. For it is found that the body radiates its heat less in moist than in dry air, so that a person feels as warm at a lower temperature when the air has a proper supply of moisture, as in a much higher temperature of dry air. Of course, less fuel is needed to warm a house when water is evaporated in stove and furnace-heated rooms. It is said by those who have experimented, that the saving in fuel is twenty per cent. when the air is duly supplied with moisture.
There are other difficulties connected with furnaces which should be considered.
The human body is constantly radiating its heat to walls, floors, and cooler bodies around. At the same time, a thermometer is affected in the same way, radiating its heat to cooler bodies around, so that it always marks a lower degree of heat than actually exists in the warm air around it. Owing to these facts, the injected air of a furnace is always warmer than is good for the lungs, and much warmer than is ever needed in rooms warmed by radiation from fires or heated surfaces. The cooler the air we inspire, the more oxygen is received, the faster the blood circulates, and the greater is the vigor imparted to brain, nerves, and muscles.