Hot air

“ Hot air goes up, so it is always warmer near the ceiling. You remember how warm air carries vapor up to make rain? It is the hot air that carries smoke up from chimneys. Did you ever send up a red paper balloon on the Fourth of July? You lit a tiny candle at the bottom. Soon the sides of the balloon swelled and stretched tight. Air expands when it is heated. That is, ittakes less hot air to fill anything than it does cold, so it grows lighter. ”

“ A. Blowing causes the air (which covers the hot food) to change more rapidly; in consequence of which, the hot air is quickly blown away, and gives place to fresh cold air.
Q. Why do ladies fan themselves in hot weather?
A. By the action of the fan, fresh particles of air are perpetually brought in contact with the face, and every fresh particle of air absorbs some heat from the skin.
Q. Does the fan cool the air?
A. No; it makes the air hotter, by imparting to it the heat out of our face: but it cools the face blown upon, by transferring its heat to the air. ”

“ We, ourselves, perspire freely, and therefore use only the hot air, but as Sir Erasmus says: “The great purpose to be arrived at, so far as temperature is concerned, is to obtain one which shall be agreeable to the sensations.” ”

“ But although my sense and reason inform me, that air in its own nature or essence is neither hot nor cold, yet it may become hot or cold, by hot or cold motions; for the sensitive perceptive motions of Air may pattern out heat or cold; and hence it is, that in Summer, when as heat predominates, the air is hot; and in Winter, when as cold predominates, the air is cold. But, perhaps, you will say, air may be cooled by moving it with a Fan, or such like thing which can make wind; wherefore it follows, that air must needs be naturally cold. ”

“ Within any given room or other enclosure hot air may rise on one side, flow over to the opposite, cool and return, and the circulation be kept up indefinitely without any necessary change in pressure. ”

“ Heat may be separated from the smoke as well as from the light, by means of a plate of iron, which will suffer heat to pass through it without the others.
3. Fire sends out its rays of heat as well as rays of [228] light, equally every way; but the greatest sensible heat is over the fire, where there is, besides the rays of heat shot upwards, a continual rising stream of hot air, heated by the rays shot round on every side. ”

“ The air from B—"
"Do we begin all over again now?"
"More or less, yes."
"So that at this place, A, it's always raining or just going to rain?"
"Yes, if it starts by being hot there, as it did just now, I suppose it is."
"What happens if it starts by being cold?"
"It rains. I've explained that. The cold air can't contain so much moisture—"
"Don't begin that again. What about B? Is it any good going there? We had frightfully high pressure there at one time." ”

“ They are both made up of tiny particles of water.
“We have learnt that the streak of cloud left by the steam-engine melts away quickly on a hot day, also that the puff is farther from the funnel on a hot day. This shows us that the warmer the air is, the more water can it take up and hold. We have also learnt that warm air is light and rises up. [2]
“What happens when warm air, which holds much water-gas, rises up to the higher and colder parts of the sky?” ”

“ A. Because the volatile gases are [Pg 49] quickly consumed; and the solid carbon plentifully supplied with air, to make it burn bright and intensely.
Q. Why does a fire burn more intensely in winter than in summer time?
A. Because the air is colder in winter, than in summer-time.
Q. How does the coldness of the air increase the heat of a fire?
A. For two reasons: 1st—Because cold air being more condensed than hot air, contains a greater body: and
2ndly—Cold air rushes more quickly to the fire, and supplies more oxygen.
Q. Why does the sun, shining on a fire, make it dull, and often put it out? ”

“ That this heat is furnished by the air, he proves, by finding, that that which we inspire contains more heat than that which we expire, or than the aqueous humor which we expire along with it, in a very considerable proportion; so that if the heat contained in the pure air did not become latent in the blood, it would raise its temperature higher than that of red-hot iron. ”

“ The atmosphere, then, is heated or cooled according as it lies upon a warm or cold part of the earth’s surface; and, by means of its aqueous vapor, it serves to store up and distribute this heat, keeping the earth from such extremes of climate as would otherwise prevail.
The air lying next to a hot surface is heated; the air touching a cold surface is cooled. And upon such differences of temperature in the air the formation of winds depends.
Hot or warm air is lighter than cold air. You have learned how heat expands bodies. ”

“ When there has been a hot sunshiny day, and when the ground has been wet, the sun attracts a great deal of vapour: then that vapour must have room, so it must push away some air to make room for itself; besides, vapour swells with heat, so it must have a great, great deal of room as it grows hotter, and hotter; and the moving the air to make way for it, must make wind. ”

“ A. The salt of the sea retards evaporation; and (as the heat of the body is drawn off very gradually) the sensation of cold is prevented.
Q. Why does sprinkling a hot room with water cool it?
A. The heat of the room causes a rapid evaporation of the sprinkled water; and as the water evaporates, it absorbs heat from the room, and cools it.
Q. Why does watering the streets and roads cool them?
A. The hot streets and roads part with their heat to promote the evaporation of the water sprinkled on them.
Q. Why does a shower of rain seem to cool the air in summer-time? ”

“ The hot day had warmed the air close to the pavement, but the cooling of the surface at the onset of darkness had caused a layer of warm air to be sandwiched in between the cold air close to the road surface and the cold air above. ”

“ A. A hot room is cooled more quickly by opening the lower sash; because the cold air can enter more freely by an under current, than by one higher up.
Q. Why does wind dry damp linen?
A. Because dry wind (like a dry sponge) imbibes the particles of vapour from the surface of the linen, as fast as they are formed. ”

“ When hot water and cold water are mixed, the hottest part of the mixture will be found to be at the surface above;—and when cold air is admitted into a warmed room, it will always be found to take its place at the bottom of the room, the warmer air being in part expelled, and in part forced upwards to the top of the room. ”

“ A. After a hot day in summer or autumn, with the wind in the west.
Q. Why is dew distilled most copiously after a hot day?
A. Because the surface of the earth radiates heat very freely at sunset; and (becoming thus much colder than the air) chills its vapour, and condenses it into dew.
Q. Does not air radiate heat, as well as the earth and its various plants?
A. No. The air never radiates heat, nor is the air itself made hot by the rays of the sun.
Q. How is the air made hot or cold?
A. By convection of hot or cold currents. ”

“ To that close, clouded, hot, narcotic room?
240
When, midnight past, the very candles seem
Dying for air, and give a ghastly gleam;
When curling fumes in lazy wreaths arise,
And prosing topers rub their winking eyes; ”

“ You know a great many times when you go out into the wind how cold it feels, and yet if the wind would actually stop, you would think it warm. It is the wind that causes the rapid evaporation and makes the surface of the skin feel cold. As it is true that the moisture in the air acts as a blanket to us in our homes, it is likewise as true that the vapor in its natural form outside of the house acts as a blanket for the earth. ”

“ When the decomposition of vapour, from the decline of the heat, is as yet but in commencement, or while the electricity remaining in the air continues to give buoyancy to the suspended particles, a delicious calm often prevails for many days in succession, amidst a perfect sunshine, mellowed by the vaporous air, and diffusing a rich golden tint, as the day declines, upon the landscape. ”

“ Well, so this water-laden air rises up, its particles, no longer so much pressed together, begin to separate, and as all work requires an expenditure of heat, the air becomes colder, and then you know at once what must happen to the invisible vapour, — it will form into tiny water-drops, like the steam from the kettle. And so, as the air rises and becomes colder, the vapour gathers into the visible masses, and we can see it hanging in the sky, and call it clouds. ”

“ The smoke, rising from several points at once, hangs in the air and collects into a dense canopy of cloud, making the heat still more stifling, till, at last, sheet-lightnings begin to play about it, followed by low growls of thunder, and, perhaps, by the blessed relief of a shower. ”

“ This heat, though it produces little change on the temperature of the air which it traverses, affords us some compensation for the heat which we radiate to the heavens. At night, also, if the sky be overcast, some compensation will be made to us, both in the town and in the country, though in a less degree than during the day, as the clouds will remit towards the earth no inconsiderable quantity of heat. But on a clear night, in an open part of the country, nothing almost can be returned to us from above in place of the heat which we radiate upward. ”

“ However much you may augment the fire, the cold air becoming heated will rise, and fresh cold air will at once take its place; and therefore no equal distribution of warmth in the house will be attained. ”

“ A. If the night has been very calm, a rapid radiation of heat has taken place in the earth; in consequence of which, the air (resting on the earth) is made so cold, that its vapour is chilled, and condensed into a thick mist. ”

“ This warming and cooling of the air is well illustrated by what takes place in a dwelling-house. If you pass out of a warm room, on a winter’s day, into the open air when there is no wind, you feel a sensation of cold. Whence does this sensation come? Not from anything you can see, for your feet, though resting on the frozen ground, are protected by leather, and do not yet feel the cold. It is the air which is cold, and which encircles you on all sides, and robs you of your heat; while at the same time you are giving off or radiating heat from your skin into the air. ”

“ Under some conditions, the warm air of a room does deposit moisture on its cooler walls. This happens in churches, concert-rooms, etc., when they are but occasionally used in winter time, and mainly warmed by animal heat, by congregational emanations of breath-vapor, and perspiration—i.e., with warm air supersaturated with vapor. ”

“ There are but few days in summer when there is not a haze in the atmosphere, although we call the sky clear, which intensifies the light and gives everything a warmer tone. The heat coming from a full moon on a clear night is absorbed in causing the aqueous vapors that are partly condensed in the higher regions of the atmosphere, to be reabsorbed into transparent vapor. ”

“ I say, some sorts: and certainly, the strongest Winds are made of the grossest Vapours. As for Smoak, it is apt to turn into some sorts of Lightning; I say, apt: for, both Vapour and Smoak can turn into many sorts of Metamorphosed Elements. As for Wind, it proceeds either from Rarified Vapour, or Contracted Air. And there are many sorts of Vapours, Smoaks, and Winds; all which sorts of Vapours and Smoaks, are apt to ascend: but, Wind is of a more level action. As for Clouds, they cannot be composed of a Natural Air; because Natural Air is too rare a Body to make Clouds. ”

“ Therefore, when we see the air is made impure by burning candles or gas lights, owing to its exceeding heat, the foul air is mostly at the top of the room, and especially when the fresh air enters cooler than the air in the room. ”

“ So, as the cooler air from above drops down, it acts like a mirror to the breath of the earth or the air that lies close over it; and this air is condensed into mist. All through the night both air and earth are cooling, but the earth more slowly, so that there is still a meeting of cooler and warmer air and consequent condensations, and the mist is hovering over the meadows when the next morning’s sun rises. As the sun mounts up, it begins to spread its warmth and the upper air is the first to feel it. Growing warm, it rises, drawing up after it the cooler air below. ”

“ We do, in fact, see this action evidenced; when in winter time the cold condenses the vapour of the breath, we see the little cloud constantly rising before the breather's face, and dispersing in the surrounding air. ”

“ Do you observe that vast sheet of white vapor that is rising from yonder valley? It is rising in consequence of the warmer air that is produced by the rising of the sun. Do you not feel that the shining of the sun upon you makes you warmer? ”

“ That which happened on a small scale, with a violent change of temperature, in the case of the alcohol fire, is taking place on a larger scale, with milder changes in temperature, all over the world. The heating by the sun in one locality causes an expansion of air at that place, and cooler, denser air rushes in to fill the partial vacuum. ”

“ How is heat diffused through the atmosphere?
By convection. The warmth radiating from the surface of the earth warms the air in contact with it; the air expands, and becoming lighter, flies upwards, bearing with it the caloric which it holds, and diffusing it in its course. ”

“ It is in winter, however, that the greatest care is required in providing a constant supply of pure air. If we would but accustom our minds to comprehend, readily and quickly, that cold air falls and warm air rises, it would assist us in our conclusions. ”

“ The benignant vapours cool our climates in summer by rendering latent the excessive heat of the noonday sun; and they temper them in winter by rendering sensible and restoring again to the air, that self-same heat. ”