Ice.
What is ice?
Ice is frozen water.
When does water freeze?
Water freezes when its temperature is reduced to thirty-two degrees.
Is water increased in bulk by freezing?
Water is increased in bulk by freezing.
Why do water-pipes often burst in cold weather?
The water in them freezes, and as it increases in bulk, the pipes burst.
Why are some springs never frozen?
Because water, much above the freezing-point, is always flowing into them from the earth, and thus they are kept too warm to freeze.
Why are some other springs frozen?
Because the water flowing into them comes from near the surface of the earth, and is soon made cold enough to freeze.
Does salt water freeze at the same temperature as fresh water?
No; fresh water freezes at thirty-two degrees, but salt water requires a lower temperature depending upon the amount of salt it contains.
How is this useful?
The surface of the ocean is never frozen over except in very cold latitudes; hence, vessels may sail on it at all seasons of the year, and carry the products of one country to another; thus adding much to our comfort and our enjoyment.
CHAPTER VI.
Sound.
When the hands are brought quickly together, what is produced?
They produce a shaking or vibration in the air.
What is this vibration in the air called?
It is called a wave in the air.
What is this wave in the air like?
It is like a wave in the water.
When this wave in the air reaches the ear, what does it produce?
It produces a sensation which is called sound.
When a stone is dropped into the water, what does it produce?
It produces waves in the water.
How do these waves move?
They move in every direction from the stone.
Do they get smaller the farther they go?
They do get smaller and smaller, until at last they disappear.
Do the waves of sound in the air act in the same way as the waves in the water?
They do act in the same way.
What then becomes of the waves of sound?
They get smaller and smaller the farther they go, until at last there is no wave left.
When the wave in the air is large, what kind of sound is made?
It makes a loud sound.
As the wave gets smaller, does the sound grow fainter?
Yes; the smaller the wave the fainter the sound, until at last both the wave and the sound die away.
Where must we be to hear a loud sound?
We must be near the place in which the sound is made.
What kind of waves will make a faint sound?
Small waves in the air make a faint sound.
Will these waves extend as far away as larger ones?
No; as all waves of sound decrease in size as they recede, the smallest ones will soonest disappear.
Why will a loud sound be heard farther than a fainter sound?
Because the waves in the air from the loud sound will extend farther away than those from the fainter one.
How are sounds produced?
Sounds are produced by striking solid bodies together.
How else may sound be produced?
By striking a solid body against a liquid, as a stick struck against the water.
In what other way may sound be made?
Sound may be made by causing a substance to move rapidly through the air.
When a piece of wood attached to a string is swung rapidly around, what kind of a noise is made?
It makes a humming noise.
When lightning passes through the air, what noise is made?
A noise which we call thunder.
How can a murmur be produced?
When sounds, too faint to be heard separately, are united, they produce a murmur.
Where may we hear a murmur?
We may hear a murmur from the brook, as its waves flow over the pebbles, and from the wind, as it passes through the trees.
When a sound is heard but once, what is it called?
It is called a noise.
When sounds are made at irregular intervals, what are they called?
They are called noises.
When sounds are repeated at regular intervals, what do they become?
They become musical sounds.
Upon what do the high and low sounds in music depend?
They depend upon the rapidity with which the waves are produced in the air.
When the waves are repeated rapidly, what sound is made?
They make a high or acute sound.
When they are repeated more slowly, what sound is made?
They make a low or grave sound.
How small a number of vibrations or waves in the air can be heard?
Sixteen vibrations in a second of time, will produce the gravest sound that can be heard.
How large a number of vibrations in the air can be heard?
Twenty-four thousand vibrations in a second of time, make the most acute sound that can be heard.
How is the human voice made?
It is made by means of the vocal chords at the upper part of the larynx.
What is the larynx?
The upper part of the windpipe is called the larynx.
How do these chords produce the voice?
When the air is forced out of the lungs, it causes these chords to vibrate, so as to produce the voice.
Are these chords the same in all persons?
No; they are larger and coarser in some persons than in others.
In whom are they larger and coarser?
They are usually larger and coarser in men, and they are smaller and finer in women.
What kind of a voice is made by the large and coarse chords?
They make a rough, coarse voice.
What kind of voice is made by the finer chords?
They make a finer and more delicate voice.
Do these chords vibrate so rapidly in man as in woman?
No; they do not vibrate so rapidly in man as in woman.
What kind of voice do rapid vibrations make?
They make an acute voice; hence, woman can, when singing, raise her voice to a very high pitch.
Where is the sensation of sound produced?
The sensation of sound is produced on the ear.
Do people vary in their ability to hear?
Yes; they vary very much in their ability to hear.
Is every person able to distinguish musical sounds?
No; some persons cannot tell one musical note from another, and some cannot even tell one bird from another by its notes.
Is the hearing of all animals alike?
No; some can hear more acutely than others; a lion has more acute hearing than a man.
What other animals have acute hearing?
All such animals as hares, rabbits, and deer.
Of what use is the acute hearing to these animals?
It enables them quickly to hear the approach of their enemies, so that they may run away in time to escape the danger.