AIR IN MOTION.
[Illustration]
[Sidenote: How a ship is moved along by air.]
The air, when it is in motion, does a great deal of work for us. It
pushes along the ships in the water. Perhaps you think that it hardly
sounds right to say that the air pushes the ships; but it really does
push them. The sails are large, broad handles for the air to press
against in pushing the vessels along in the water. On the preceding
page is a ship with many sails, and most of them are unfurled, or put
out for the breeze to press upon.
The air would push a vessel along to some extent, even if there were
no sails, by pressing or blowing against the body of the vessel; but,
unless the wind blew very strong, the air would not push it along very
fast in this way. And so sails are put up on masts, that more of the
air may get hold, as we may say, so as to press on the vessel.
[Illustration]
[Sidenote: A coat used as a sail.]
Sometimes the wind helps you along as you are walking. Now, if you
take hold of your coat, and spread it out wide, as you see this boy
is doing, it will be like a sail, and the wind will carry you along
faster, because there is more for the air to press upon. So, too, if
you have an umbrella open when the wind is blowing on your back, it
will be to you as the sail is to the ship. But if you are going against
the wind, the outspread coat and the open umbrella would prevent your
getting along fast.
[Sidenote: Trees blown by the wind.]
When a tree is bare, the wind scarcely moves its branches; but how it
bends when it is full of leaves and the wind blows strongly upon it! It
is then like a ship with its sails all unfurled; there is a great deal
for the air to press upon.
[Sidenote: Fast-moving air.]
Sometimes we say the wind blows very hard or very strong; this is when
the air moves very fast. The faster it moves, the more it will do. This
is so with other things. When you strike any thing very hard with a
stick, you do it by making the stick move fast. When there is only a
gentle breeze, that you can just feel, the air is moving very slowly;
it is like the gentle touch with the stick. But when the wind blows so
hard that you can scarcely stand up, the air is moving very fast.
[Sidenote: The bullet.]
If a bullet is tossed to you, it will not hurt you to catch it, because
it does not move very fast; but if a bullet shot from a gun should hit
your hand, it would wound it, and perhaps go through it. The reason is,
that the bullet moves so fast. The faster it moves, the more harm it
will do. So the air, when it moves very fast indeed, is apt, like the
bullet, to do harm.
[Sidenote: The locomotive.]
You have seen a locomotive backed up against a train of cars to be
hitched on. It does no damage, because it is backed up slowly. It only
gives a little jerk, you know, to the whole train. Now, if it moved
very fast, it would, when it came to the cars, break them to pieces. It
is for the same reason that fast-moving air roots up trees, blows down
houses, and drives ships on shore, dashing them against the rocks.
[Sidenote: Ship in a storm.]
When the wind blows hard, the sailor takes in some of his sails. The
vessel would go too fast if he left them all out, because there would
be so much for the air to press on. If the wind blows very hard indeed,
he takes down all the sails, fastening them very tightly, so that the
wind may not loosen them. Even with all the sails down the ship will
go quite fast enough, perhaps even too fast, pushed along by the wind
that strikes right upon it. Here is a ship in a storm. You see how the
sailors have tied up most of the sails. One of them has been torn from
its fastenings by the violence of the wind, and is in tatters.
[Illustration]
[Sidenote: How waves are made.]
The waves that you sometimes see rise so high are made by the striking
of the air upon the water; and the faster the air moves over the water,
the higher they rise. When the air is very still there is scarcely a
ripple, and the water looks like smooth glass; and you would hardly
think, as you look upon it, that such a light thing as air is could
whip it into such waves as you sometimes see.
The waves in the ocean are much higher than they are in a river. This
is because the wind blows over so much greater an extent of water in
the ocean.
[Sidenote: Small and great whirlwinds.]
You have heard of whirlwinds. In these the air moves in a whirling way
instead of straight forward. You sometimes see little whirlwinds in the
street; and as shavings and other light things are whirled about in
them, and are carried up in the air, you can imagine what damage large
whirlwinds can do, twisting up trees and tearing houses in pieces.
As you can not see the air, and it is a very light thing, you commonly
think of it as being almost nothing, and yet it does these great things
that I have mentioned. When we see this light thing raise the waves,
and move the heavy ships along so swiftly, we see that there is great
power in it.
_Questions._--How does the air make a ship go? What is the need of
sails? What is said about the air’s helping you along in walking? Why
does the wind bend a tree so much that is covered with leaves? What is
true about the air when the wind blows hard? Give the comparison about
the stick, the bullet, and the locomotive. Why does the sailor take
down some of his sails when the wind blows hard? What is said about
waves? Why are they higher in an ocean than in a river? What is said
about whirlwinds?