Winds.
What is wind?
Wind is air moving from one place to another.
Does the air move in every direction?
It does move in every direction, upward, downward, and to every side.
When the wind blows from the north towards the south, what is it called?
It is called a north wind.
What is it called, when it blows from other directions?
When it blows from the east, it is called an east wind; from the south, a south wind; and from the west, a west wind.
What causes the air to move from place to place?
Heat causes the air to move from place to place.
When air is heated, what change takes place?
The heated air ascends, and colder air moves in to fill its place; thus causing the wind to blow.
What illustration have we of this?
When a candle is held in the open door of a heated room, its flame is blown outwards if placed at the top of the door, and inwards if placed at the bottom of the door; thus showing two currents of air.
Why is this?
The heated air escapes from the top of the room, so as to ascend higher, while the colder air comes in at the bottom, to fill its place.
Why does the wind blow towards a fire in the open air?
Because the air near the fire becomes heated, and ascends rapidly, so that the air rushes in from every direction to fill its place.
What does the earth do with the heat it receives from the sun?
The earth gives off its heat to the air nearest to it.
What becomes of this heated air?
This heated air ascends, and colder air comes to take its place; thus producing winds.
Are all parts of the earth heated alike by the sun?
No; they are not all heated alike.
How does this unequal heating affect the air?
It causes currents in the air all over the world.
Does water give off heat as rapidly as the earth does?
No; water is a poorer radiator of heat than the earth is.
On which will the air be the cooler when the sun shines, on the earth, or on the water?
It will be cooler on the water than on the earth.
Why does a sea-breeze feel cool in the daytime?
Because the air over the water, in the daytime, is less heated than the air over the land; hence, the sea-breeze feels cool.
Why does the earth become colder than the water at night?
Because the earth gives off its heat more rapidly than the water, and as it receives no heat from the sun at night, it soon becomes colder than the water.
Why does a land-breeze feel cool at night?
Because the air over the land at night is less heated than the air over the water; hence, the land-breeze feels cool.
How rapidly does the wind move?
A gentle breeze goes four or five miles in an hour; a high wind goes from twenty to fifty miles in an hour, and a hurricane goes from sixty to one hundred miles in an hour.
Of what use is the wind?
The wind carries away the impure air and injurious gases from cities and other places, so that they are the more healthy; it dries up the water on the surface of the earth, and thus promotes vegetation; it sweeps the clouds filled with moisture from the lakes and the oceans, over every part of the world, so that the water in them may fall in refreshing showers, to fill the springs, to purify the air, and in many other ways to make the earth a fitting place of abode for man. But these are only a few of the many blessings that an all-wise Creator has given us; and, for them all, we should ever remember to offer up to Him the homage of a grateful heart.
CHAPTER V.
Water.
Where do we find water?
We find water almost everywhere; above the earth, on the earth, and beneath the earth’s surface.
Where do we find water above the earth?
We find it in the air, in the form of vapor.
Where do we find water on the earth?
We find it in springs, in streams of water, and in lakes and oceans.
Where else is water on the earth?
It is in the sap of plants, the juice of fruits, and the blood of animals.
Where do we find water beneath the earth’s surface?
We find it in streams running in every direction.
Has water any influence on climate?
Yes; water causes many changes in climate.
What changes does water produce in climate?
It makes spring and summer cooler, and autumn and winter warmer.
How does water make the Spring of the year cooler?
The ice and snow while melting, take up a large amount of heat, which passes into the water as latent heat.
Where does the melting ice and snow obtain this heat?
They obtain it from the air, and from objects near the surface of the earth.
What advantage is derived from this?
The tender buds and blossoms are kept back in the spring until the ice and the snow are melted, and the earth becomes warmed, so that they are seldom injured by the cold.
How does water make Summer cooler?
By the streams, lakes, and oceans, which absorb heat from the sun; and, as the water is a poor radiator, but little of this heat is given off to the air.
What effect does this heat produce upon the water?
The temperature of the water is gradually increased by it during the hot weather of summer.
How is this useful?
It gives heat to the plants in the water, so that they may grow; and it rouses into active life, the fish and other breathing things, from the torpid state in which they are during winter.
Does water store away a large amount of heat?
It does; and thus the summer is made much cooler.
How does water make Autumn warmer?
In the autumn the heat in the water is given off, so that the air is kept warmer than it would otherwise be.
How does this affect autumn weather?
It makes that season of the year mild and pleasant.
What advantage is derived from this?
It gives the late fruits and seeds a chance to ripen; and it affords time for the sap of plants to return to the roots, and to become prepared for the frosts of winter.
How does water make Winter warmer?
By being changed into ice and snow.
When water is changed into ice or snow, what takes place?
The latent heat in the water is given off, and thus the winter air is made warmer.
Can water be compressed into a smaller bulk?
Yes; it can be compressed, but not so much as air.
What is said of water that is compressed?
Its particles are pressed closer together, and it is said to be more dense.
Why is the water at the bottom of a pond more dense than that at the surface?
Because the water at the bottom of the pond has to bear the weight of all the water above it.
Is the weight of the water very great?
The weight of the water is very great.
What is its weight at the depth of one hundred feet?
At the depth of one hundred feet, it presses in every direction, with a force of forty-three pounds on every square inch of surface.
What is its weight in the ocean at the depth of one mile?
At the depth of one mile it presses in every direction, with a force of two thousand three hundred pounds on each square inch of surface.
When one opening is made near the top, and another near the bottom of a vessel full of water, from which will the water flow with the most force?
From the opening near the bottom of the vessel.
Why is this?
Because there is more weight of water to press it out from the opening near the bottom.
When water is poured into a vessel, what becomes of it?
It spreads out so that its surface is level.
When different vessels are connected together, what does the water do?
It falls to the same level in all of them.
How high will the tea stand in the spout of a tea-pot?
Just as high as the tea is in the tea-pot.
Fig. 20.
Will the tea in the spout balance that in the tea-pot?
The tea in the spout will balance that in the tea-pot, as is shown in [Fig. 20].
How else can it be shown that water will rise to the same level in different vessels?
By a reference to [Fig. 21], this may be seen.
Fig. 21.
Let these vessels be made so as to connect with each other by the tube at the bottom; then, when water is poured into one of them, it will rise up in each of the others, so as to be the same height in all.
Where is this principle used to benefit mankind?
It is used in supplying the inhabitants of cities with water.
What must be built in order to supply a city with water?
A reservoir must be built.
What is a reservoir?
It is a large basin made so as to hold water.
How high is the water in the basin?
It is usually as high as the tops of the houses in the city.
How is the water raised into the basin?
It is raised into the basin by means of pumps.
How is the water conveyed from the basin to the houses in the city?
By means of pipes laid under the surface of the earth, and extending to all parts of the city.
How high will the water rise in these pipes?
Nearly as high as it is in the basin.
If an opening be made in a pipe, how high will the water be forced up?
Nearly as high as the water in the basin.
What is the water flowing from such an opening called?
It is called a fountain.
What makes the water shoot up in a fountain?
The pressure of the water in the basin forces it to shoot up in the fountain.
To what does water always tend?
Water always tends to fall to a level.
What instrument is constructed on this principle?
The water-level.
What is a water-level?
It is a glass tube so nearly filled with water that it contains only a bubble of air. This tube is fastened in a piece of wood, as shown in [Fig. 22].
Fig. 22.
How can we tell when the instrument is level?
By the bubble of air, which will always be at the centre of the tube when it is level.
What other liquid may be used to fill the tube?
Alcohol may be used to fill the tube, and it is then called a spirit-level.
Of what use is the spirit-level?
It is used for levelling the walls and wood-work of houses, bridges, and other structures.