Does the sun radiate heat?
The sun does radiate or give off both heat and light.
How do heat and light from the sun come to us?
They come together in the sunbeam.
Does heat travel as rapidly as light?
Yes; heat and light travel together at the rate of 200,000 miles in a second of time.
Do all substances radiate heat equally well?
No; some substances are much better radiators than others.
What kind of objects are the best radiators?
Those with rough and dark surfaces.
What kind of objects are the poorest radiators?
Those with smooth and bright surfaces.
Why does water keep hot a long time in a bright tin vessel?
Because the tin is a poor radiator, and prevents the heat of the water from passing off into the air.
Why does water soon cool in a dull earthen vessel?
Because the earthen vessel is a good radiator, and gives off the heat of the water rapidly.
If the bright tin vessel be painted, will the water keep hot so long?
No; the tin becomes a good radiator when painted, and the water is soon cooled.
Why does a stove cool so quickly?
Because the iron is a good radiator, and gives off its heat rapidly.
Do the rays from the sun give off much heat while passing through the air?
The rays from the sun give off about one-fourth of their heat while passing through the air.
What becomes of the remaining heat from the sun?
It is absorbed or reflected at the earth’s surface.
What does the earth do with the heat?
The earth radiates the heat, and thus warms the air.
When a piece of bright tin is held near the fire, does the tin become hot?
No; the tin is scarcely warmed.
What does the tin do with the rays of heat from the fire?
The tin throws them back from its surface.
What is this called?
It is calledReflectionof heat.
Do all substances reflect heat equally well?
No; some reflect much more heat than others.
What kind of objects reflect heat the best?
Those with bright and smooth surfaces.
What kind of objects reflect heat the least?
Those with rough and dark surfaces.
Does bright tin reflect heat better than glass?
Yes; bright tin reflects about eight times as much heat as glass does.
Why does it take so long to heat water in a new tin cup?
Because the bright tin is a good reflector, and throws off the greater part of the rays of heat that come to it from the fire.
Why does a piece of tin or other bright metal, placed under a stove, protect the carpet from burning?
Because the tin being a good reflector, remains cool, and cannot burn the carpet under it.
Why will not a piece of iron protect the carpet from burning?
Because the iron, being a poor reflector, soon becomes heated, so as to burn the carpet under it.
When a piece of iron is held near a fire, does the iron become hot?
Yes; the iron soon becomes hot.
How is the iron heated?
The iron absorbs or takes in the heat from the fire.
What is this power to take in the heat called?
It is calledAbsorptionof heat.
What is this process like?
It is somewhat like a sponge, that absorbs or sucks up moisture.
Do all substances absorb heat equally well?
No; some substances absorb more heat than others.
What kind of objects absorb heat the best?
Those with rough surfaces absorb heat the best.
What kind of objects absorb heat the least?
Those with bright and smooth surfaces.
What colored objects absorb heat the best?
Objects of a dark or black color.
Do objects that absorb heat well, radiate well?
Yes; good absorbers are good radiators.
Why is this?
Because every substance must absorb heat before it can have heat to give off.
Do objects that absorb heat well, reflect it well?
No; objects that absorb heat well, cannot reflect it well.
Why is this?
Because a ray of heat cannot be reflected from an object, and be absorbed by that object, both at the same time.
Why is water rapidly heated in a tea-kettle?
Because the rough, dark iron of the tea-kettle absorbs the heat from the fire, and gives it off rapidly to the water contained in the kettle. The iron is also a good conductor of heat; therefore, the heat from the fire passes rapidly through it to the water.
Why does black cloth, when spread on the snow, cause the snow to melt more rapidly?
Because the cloth absorbs heat from the rays of the sun, and, therefore, melts the snow beneath it.
Why does not white muslin, when spread on the snow, hasten its melting?
Because the white muslin reflects the heat from the rays of the sun, so that but little of it enters the snow.
Does the snow receive much heat from the sun?
Snow is a good reflector, and throws back most of the heat from the sun.
What causes the snow to melt?
The heat it receives from touching the earth, causes the most of it to melt.
Why does snow melt first on dark earth?
Because dark earth is a good radiator, and gives off much heat to the snow.
Why does snow remain longest on light-colored earth?
Because the light-colored earth is a poor radiator, and gives off but little heat to the snow.
Why does the air feel cold in winter time?
Because it is so much cooler than our bodies that it absorbs heat from them rapidly, and thus makes them feel cold.
Why does the air feel warm in summer time?
Because it is then nearly as warm as our bodies, and, therefore, absorbs heat from them slowly.
Why are greasy shoes warmer than those polished with blacking?
Because the greasy shoes absorb heat from the sun, but the polished shoes throw off the heat of the sun by reflection.
What may we remember about heat?
And God said, Let there be light; and there was light. But along with the light, came its companion—heat; for when the sunbeam first came forth at His command, the twin sisters, light and heat, together touched the earth, causing it to bloom with life and beauty. And the earth brought forth grass, the herb yielded its seed, and the tree its fruit: And God saw that it was all good.
An outdoor walk.
Decoration.
Illustrated Capital W
Where do we find air?
We find air everywhere around the earth.
Does the air fill every place?
The air fills every place not filled by some other substance.
When we say a bottle is empty, what do we mean?
We mean that it does not contain anything but air.
What do all vessels contain when empty?
All vessels contain air when seemingly empty.
Do all liquids contain air?
Yes; all liquids contain air.
Where is the air in a liquid?
The air is among the particles which compose the liquid.
Is there air in water?
Yes; there is air in water.
Of what use is the air in the water?
It is used by the fish and other creatures that live in the water.
How do the fish and other creatures use it?
They breathe it into their lungs, and it supports life in them, the same as the air which we breathe supports life in us.
Do solids contain air?
Yes; solids contain more or less air.
Where is the air in fur and in wool?
It is among the hairs of the fur and the wool.
Where is the air in cotton and linen goods?
It is among the fibres of the cotton and the linen.
Where is the air in fruit?
It is among the particles composing the fruit.
Where is the air in wood?
It is in the pores or small spaces in the wood.
Is there air in the earth?
Yes; there is air among the particles of the earth.
How deep in the earth does the air extend?
We cannot tell; but both air and water have been found several hundred feet below the earth’s surface.
Of what use is the air in the earth?
It is breathed by the worms and other creatures that live in the earth.
When part of the air is taken from a room, what becomes of the air left in the room?
It expands so as to fill the whole room.
Are the particles of air so close together after it expands as they were before?
They are not so close together.
When air is thus expanded, what is said of it?
It is said to berare.
When pressure is put upon air in a vessel, what becomes of the air?
It is forced or pressed into a smaller space, and, therefore, occupies less room.
When pressure is put upon air, are its particles closer together?
They are closer together.
When air is thus compressed, what is said of it?
It is said to bedense.
Can air be much condensed by pressure?
It has been condensed so much that 100 cubic feet of air was reduced to the bulk of only one cubic foot.
What makes the air so dense as it is at the surface of the earth?
The pressure of the air above the surface.
Does the air then become less dense the higher we ascend?
Yes; because there is less air above to press upon it.
Does the air become more dense the deeper we go in the earth?
It does; because there is more air above to press upon it.
How can we understand this?
In a large heap of wool, the hairs of the wool at the bottom of the heap are pressed closely together, but they are farther and farther apart the nearer they are to the top of the heap.
How high does the air extend above the earth?
It is supposed to extend to the height offiftymiles.
Do these fifty miles of air press upon the earth?
They do press upon the earth.
With how much force does the air press on the earth?
With a force offifteenpounds on every square inch of surface.
Then has the air weight?
The air has weight.
Why do we not feel the weight of the air?
Because it presses with equal force, in every direction; up, down, and on every side.
How can we show that the air presses upwards?
Fill a tumbler with water and put a piece of writing-paper over the top. Hold the hand on the paper and quickly invert the tumbler, or turn it bottom upwards; then remove the hand, and the pressure of the air against the paper will keep the water in the tumbler. This is represented inFig. 12.
Fig. 12.
Fig. 12.
If a tumbler be filled with water in a bucket, and then raised bottom upwards until its edge is just below the surface of the water, it will remain full of water; why is this?
The pressure of the air on the water in the bucket, keeps the water in the tumbler.
If we use an iron pipe or a lead pipe, instead of a tumbler, will the pressure of the air keep the water in the pipe?
It will, the same as it keeps it in the tumbler.
How high will the air keep the water in the pipe?
It will keep the water to the height ofthirty-fourfeet.
Why does the air keep the water up thirty-four feet?
Because the air presses down with a force of fifteen pounds on every square inch of surface, and a column of water thirty-four feet high presses down with a force of fifteen pounds on every square inch of surface.
Does the weight of the air exactly balance the weight of the column of water?
It does; they balance each other.
Is water heavier than air?
Water is heavier than air; because a column of water thirty-four feet high weighs as much as a column of air fifty miles high.
Is quicksilver heavier than water?
Yes; quicksilver is about thirteen times heavier than water.
How high a column of quicksilver can the air keep up?
The air can keep up a column of quicksilver only thirty inches high.
Why is this?
Because a column of quicksilver thirty inches high, weighs exactly the same as a column of air fifty miles high, or a column of water thirty-four feet high.
How can this be shown?
Take a glass tube about thirty-three inches long, cork up one end, and fill the tube with quicksilver; then put the open end of the tube beneath the surface of some quicksilver, in a basin, and the quicksilver in the tube will fall to the height of thirty inches, as is shown inFig. 13.
Fig. 13.
Fig. 13.
If the cork be removed from the glass tube, what will become of the quicksilver?
It will immediately sink into the basin below.
Why will the quicksilver sink into the basin?
Because the air will then press upon the quicksilver in the tube, the same as it does on that in the basin; hence, all the quicksilver will fall to the same level.
Does the weight of the air vary at different times?
Yes; the air does vary in weight at different times.
How can we tell when the air changes in weight?
By means of aBarometer.
What is a barometer?
It is an instrument having a tube filled with quicksilver.
When the air is heavier, how does it affect the quicksilver?
The quicksilver rises in the tube.
When the air is lighter, how does it affect the quicksilver?
The quicksilver sinks in the tube.
What is the use of the barometer?
It shows the changes about to take place in the weather.
When the quicksilver rises, what kind of weather is indicated?
It indicates fair weather.
When the quicksilver sinks, what kind of weather is indicated?
It indicates cloudy or stormy weather.
Where is the barometer most useful?
It is most useful at sea, to warn the sailors of coming storms.
What use is it to farmers?
They can use it as a guide when planting and gathering their crops, and thus be better prepared for stormy weather.
For what else may the barometer be used?
It may be used to tell the height or depth of any place.
Does the air become lighter as we ascend?
The air does become lighter as we ascend.
How does this affect the barometer?
As the air becomes lighter, the quicksilver in the barometer sinks lower.
At what height does the quicksilver stand at the level of the sea?
It stands at the height of thirty inches.
When we ascend a mountain 10,000 feet high, how much does the quicksilver sink?
It sinks ten inches, and, therefore, stands at the height of only twenty inches.
Does the same thing take place when we ascend in a balloon?
Yes; the higher the balloon ascends, the lower the quicksilver in the barometer sinks.
When we descend into the mines dug in the earth, does the air become heavier?
It does; and the quicksilver in the barometer rises.
At what temperature does water boil?
Water boils at a temperature oftwo hundred and twelvedegrees.
When the weight of the air is taken off, will the water boil at a lower temperature?
Water will then boil at a lower temperature.
Where will water boil at the lowest temperature, on a mountain, or in a valley?
It will boil at the lowest temperature on a mountain.
At how much lower temperature will water boil, as we ascend?
Water will boil at one degree less for every five hundred feet we ascend.
How high is the city of Quito, in South America?
Quito is nine thousand feet above the level of the sea.
At what temperature does water boil there?
Water boils, in Quito, at a temperature of one hundred and ninety-four degrees.
Will an egg boil hard at Quito?
No; because the water passes off into steam at one hundred and ninety-four degrees, and it requires more than one hundred and ninety-four degrees of heat to boil an egg hard.
Do persons, while ascending mountains, often have difficulty in cooking their food?
They do, on account of water boiling there at a temperature below two hundred and twelve degrees.
What is avacuum?
Avacuumis a place that does not contain anything, not even air.
When a vessel contains water and we pour the water out, does that produce a vacuum?
No; because air passes in and fills the vessel.
How then can a vacuum be produced?
A vacuum in any place, can be produced only by drawing the air out of it.
When a vacuum is produced, what does the surrounding air try to do?
It tries to get in so as to fill the vacuum.
With what force does the air try to fill a vacuum?
With a force of fifteen pounds on every square inch of surface.
When a vacuum is produced in contact with a liquid, what does the liquid do?
The liquid tries to pass in to fill the vacuum.
Can there be a vacuum in contact with a gas or a liquid?
No; because the gas or the liquid will instantly fill the vacant space.
Why does water make a gurgling noise when poured out of a bottle?
Because air is passing into the bottle at the same time that the water is coming out, and the air makes the noise in passing through the water.
Why does water make a gurgling noise when running out of a barrel?
Because the air passes into the barrel at the opening where the water passes out, and thus the noise is made.
How can the gurgling noise be prevented?
By making an opening at the top of the barrel, so as to admit the air.
Does water flow freely from a vessel that has but one opening?
No; because part of this opening is needed to admit air into the vessel.
Why does the water flow freely, when there is an opening at the top as well as at the bottom of the vessel?
Because air is admitted at the top, and there is then nothing to obstruct the flow of water from the opening at the bottom.
What causes fruit, vegetables, and meat to spoil?
The air causes them to spoil.
How do we know this?
Because when kept from the air they remain good for a long time.
What is this process of keeping fruit called?
It is called canning.
How is fruit canned?
The fruit is put into a glass jar, or into a tin can, along with some water, and then heated until the jar or can is filled with steam and the air is all forced out; it is then sealed so as to be air-tight, and, in this condition, may be kept for many months.
What becomes of the steam in the jar or can?
As the fruit cools, the steam is condensed to water, and a vacuum is produced, so that the fruit is kept from the air.
What is the plaything which boys call a sucker?
Asuckeris a round piece of leather, with a string fastened to its centre.
How is the sucker used?
The leather is made very soft and wet, so that when pressed against a stone or brick, all the air under the leather is forced out; and when the leather is lifted up, the stone or brick is lifted with it, as is represented byFig. 14.
Fig. 14.
Fig. 14.
What keeps the leather against the stone?
The pressure of the air upon the leather keeps it against the stone.
With how much force does the air press the leather against the stone?
With a force of fifteen pounds on every square inch.
When there is air between the stone and the leather, can the stone be lifted with the sucker?
It cannot; because the air under the leather will press it up as much as the air above will press it down.
How does a fly walk upon the ceiling?
The fly’s feet are much like the sucker, and are kept against the ceiling by the pressure of the air.
How do snails cling to objects?
They force out all the air between the object and their bodies, and are then held against the object, as the sucker is held against the stone.
What is asiphon?
Asiphonis a bent tube, having one branch longer than the other, as is represented inFig. 15.
Fig. 15.
Fig. 15.
For what is the siphon used?
The siphon is used to draw liquids from one vessel into another.
How can water be drawn from a vessel by means of a siphon?
Put the short branch in the water, then, by means of the mouth applied to the long branch, suck the air from it, and the water will begin immediately to flow from the long branch.
Where must the open end of the short branch be kept?
It must be kept beneath the surface of the water, so as to prevent air from entering the tube.
Where must the open end of the long branch be kept?
It must be kept lower than the open end of the short branch.
While the water is running out of the long branch, what is produced at the bend of the tube?
The tendency is to produce a vacuum at the bend of the tube.
What is the result of this?
The water is continually forced up the short branch to fill this vacuum, so that it runs out of the long branch in a steady stream.
How do boys often suck cider from a barrel?
By means of a straw.
How is this done?
One end of the straw is placed in the cider, and the other end in the mouth; then the air is sucked out of the straw, and the cider is forced up to fill the vacuum. When the cider is sucked out, more cider is forced up, so that the straw is kept full.
What makes the cider rise to fill the vacuum in the straw?
The pressure of the air on the cider in the barrel.
To what height will water rise in a tube to fill a vacuum?
The water will rise to the height of thirty-four feet.
To what useful purpose is this applied?
It is applied to makingpumps.
Of what material are pumps made?
Either of wood, or of metal, such as lead or iron.
What is necessary in regard to the tube or pump-tree?
It must always be air-tight.
Why must the tube or pump-tree be air-tight?
So that a vacuum may be produced in it.
How is this accomplished?
By means of two valves in the pump-tree.
What is a valve like?
A valve is like the lid on a tea-kettle, or the cover on an inkstand; and it is kept in its place by a hinge on one side.
Where is one valve placed?
One valve is placed in the lower part of the pump-tube.
How near the water must this valve be?
It must be within thirty-four feet of the water.
Why must the lower valve be within thirty-four feet of the water in the well?
Because the air can force water up a tube only to the height of thirty-four feet.
Does this valve move about in the pump-tube?
No; it remains fixed in one place.
Is this valve air-tight?
Yes; the lower valve or bucket in a pump must always be air-tight.
Where is the upper valve?
It is fastened on the end of a rod, which is attached to the pump-handle.
Does this valve remain fixed in one place?
No; it moves up and down when the pump-handle is moved.
Is the upper valve air-tight?
No; the upper valve is not air-tight.
Fig. 16.
Fig. 16.
Fig. 17.
Fig. 17.
Fig. 18.
Fig. 18.
Explain the working of a pump.
When the pump-handle is lowered, the water, abovethe valve or bucket attached to the end of the rod, is raised so that it runs out at the spout; and the vacuum, thus produced by raising the water, is filled by the water in the well, which is forced up the pump-tube by the pressure of the air. When the pump is at rest, the lower valve, which is air-tight, prevents the water from sinking back into the well, so that the pump-tube is always full of water, whether we are pumping or not.
What doesFig. 16represent?
It represents a pump, when it is not working.
How are the valves?
The valves are both closed.
What keeps the water above the lower valve?
The lower valve is air-tight, and, therefore, water-tight, so that it keeps the water above it.
What keeps the water in the tube below the lower valve?
The pressure of the air on the water in the well, forces it up the tube to the lower valve.
What doesFig. 17represent?
It represents a pump, when the handle is being raised.
How is the lower valve?
The lower valve is closed.
Why is the lower valve closed?
So as to prevent the water from being forced back again into the well.
How is the upper valve?
The upper valve is open.
Why is the upper valve open?
So that the water may pass up through the valve, while the valve is descending.
What doesFig. 18represent?
It represents a pump when the handle is being lowered.
How is the upper valve?
The upper valve is closed.
Why is the upper valve closed?
So as to lift up the water which is above it.
What becomes of the water which is lifted up?
It runs out of the spout of the pump.
As the upper valve is raised up, what is produced under it?
A vacuum is produced under it.
How is this vacuum filled?
The water below the lower valve is forced up, so as to fill the vacuum.
How is the lower valve?
The lower valve is open.
Why is the lower valve open?
So that the water may pass through and fill the vacuum above the valve.
Where are pumps mostly used?
Pumps are mostly used by people in the country, for the purpose of obtaining water from the wells which have been sunk in the earth.
When does smoke ascend through the air?
When smoke is lighter than the air, it ascends through it.
When does smoke descend through the air?
When smoke is heavier than the air, it descends through it.
Why does a cork rise to the surface of the water?
Because the cork is lighter than the same bulk of water; hence, it ascends to the surface.
Why does heated air ascend?
Because it is lighter than the cold air.
If a feather be put in the heated air, will it be carried up with the air?
The feather will be carried up with the air.
If warm air be confined in a bag, will it rise up and carry the bag with it?
It will rise up and carry the bag with it.
What might this bag be called?
It might be called aballoon.
What is a balloon?
It is a bag filled with gas, and used for sailing in the air.
Fig. 19.
Fig. 19.
What is placed over this bag?
A network of ropes is placed over it, and they are so arranged as to extend beneath the bag, as is represented inFig. 19.
What is fastened to these ropes?
A car or basket is fastened to these ropes.
What is the car or basket for?
It is to carry up those who wish to ascend with the balloon.
How large are balloons usually made?
They are from twenty to thirty feet in diameter.
With what are balloons filled?
Balloons are filled with a gas lighter than air.
What gas is often used for filling balloons?
The coal gas, such as is used to light our cities at night.
What lighter kind of gas is sometimes used?
Hydrogen gas is sometimes used.
Is hydrogen gas much lighter than the air?
It is so much lighter than the air, that fifteen cubic feet of hydrogen gas weigh no more than one cubic foot of air.
How much less will a balloon thirty feet in diameter weigh, when filled with hydrogen gas, than when filled with air?
It will weigh about ten thousand pounds less.
How much less, when filled with coal gas than when filled with air?
About two thousand pounds less.