THE PRESSURE OF WATER.
[Sidenote: The pressure of the particles of water upon each other.]
Any thing that is solid presses only one way, directly down; but water
or any fluid presses all ways. It presses just as much sidewise, or
even upward, as it does down. The reason is, that the particles of
water move about among each other, and are not fastened tight together
as they are in a solid. When water freezes, its particles become all
fastened together, and then the pressure is all downward.
[Illustration]
To see how this pressure of the particles of water operates, look at
some shot lying together. One shot does not lie right upon another shot
below it in this way, _a_, but they lie in this way, _b_. You see that
each shot presses down between those that are underneath it. Each shot
is trying, as we may say, to get down between its neighbors below; and
if there was nothing to prevent it, it would press them apart.
[Sidenote: The pile of balls.]
You can see that this is so by trying a little experiment. Put some
shot close together on a very smooth surface. Now put another shot on
top of them, and you will see that it will press them all apart. If the
shot should be rough, and the surface on which you lay them should be
rough also, your experiment will not succeed, because the shot will not
roll easily. It is for this reason that cannon balls, as you see them
piled up in an ordnance-yard, as represented in the annexed figure, do
not roll away. If they were smooth, and the place which they were piled
on were smooth, they would all be pressed apart, and the pile would
thus be spoiled.
[Illustration]
Now see what this sidewise pressure will do in a vessel filled with
shot if there should be an opening made in the side. The shot close by
the opening will run out, because they are pressed sidewise by the shot
lying right above them; and as they go out, those that press them out
will be pressed out in their turn by those above them, and so on.
[Sidenote: Particles of water compared with shot.]
Just so it is with the little fine balls or particles of water. They
lie on each other in the same way that shot do. Each particle is
pressing always to get down between the particles that are underneath
it, as I have showed you it is with the shot. And if you make an
opening in the vessel that holds the water, its particles will run,
or rather roll out, like the shot, only a great deal easier, because
they are so smooth. They are pushed out by this pressing down of each
particle between those that are below it.
[Sidenote: About water running from openings in different parts of a
vessel.]
If you make an opening near the top of a vessel filled with water, it
does not run out with much force; but if the opening be made near the
bottom, it spouts out as if it was in a great hurry to get out of the
vessel. What is the reason of this difference? To understand this,
observe that all the particles are pushing downward in the way that I
have shown. Those particles, therefore, that are near the bottom, have
a great deal more pressure on them than those that are near the top;
so that when the opening is made near the bottom, the particles there
are pushed out with great force. There is a large crowd of particles
pushing down to get out at that opening. And observe, as the water in
the vessel lessens, the force of the stream from the opening lessens;
it does not leap out so straight as it did at first. It is very much
as it is with a crowd pressing through a door. When the crowd is very
great, those that are pushed through the door are pushed with great
force; but as the crowd lessens, the pressure lessens.
It is found that water runs out of a vessel from an opening in the
side close to the bottom just as quickly as it does from an opening of
the same size in the bottom itself. What is the reason of this? It is
because the little round particles of water roll so easily. They roll
out just as easily as they drop out.
See the difference between pressing on a fluid and on a solid. If you
press on a block of ice, you press it all one way. If you press it
down, you press it all down. If you press it sidewise, it all moves
sidewise. And it makes no difference whether your hand, or whatever you
push with, covers the whole side of the block or not. But it is not so
with water. If you press your hand down into a vessel of water, you
press down some of the water, but not all of it. Some of it is pressed
up; for, as you press down what is right under your hand, this pushes
what is below it off each way to the side, and this pushes up the water
that is over it. This is because the round, smooth particles roll so
easily on each other. When pressure is made upon them, they roll away
from it just where they can--downward, or sidewise, or upward.
[Sidenote: Water moving in a tube.]
There is one way in which you can make all of a body of water go
straight along. It must be in a tube, so that it can not escape
sidewise, and then there must be something to fit this tube which
will push along the water. It must fit exactly, or some of the little
particles will slip back by it.
In this way you can push the round body of water in the tube straight
along, just as you push a round stick or a long icicle. But suppose
that there is a little hole in the tube. This would make no difference
if the water were ice, because the particles of a solid are so tightly
fastened together; but the pressed liquid, you know, will spout out of
the hole, because the particles, not being well fastened together, will
escape from the pressure wherever they can. Open a door any where, and
out they will leap.
[Sidenote: Squirt-guns and stick-guns.]
[Sidenote: The gas and the ball.]
You see the difference between a liquid and a solid in the operation of
a squirt-gun, and of one of the stick-guns so common among children.
So long as the water is in the squirt-gun, it is all pushed along
together, as the stick is in the stick-gun. But as soon as it gets
out, it becomes all divided up by the air, just as you saw in the last
chapter the water from a fountain does. But the stick, as it flies
out of the gun, keeps whole, because its particles are well fastened
together. If the water were changed into ice, it would fly out whole as
the stick does, for its particles would be so fastened together that
the air could not separate them as it does the particles of water.
[Sidenote: Attraction in solids, and fluids, and gases.]
The difference is still greater between solids and gases. You see this
in the firing of a gun or a cannon. The gas into which the powder
changes keeps together while it is in the gun, just as the water does
in the squirt-gun; but as soon as it gets out, it spreads like the
water when it gets out of the squirt-gun, only a great deal more. This
is because the particles of the gas are disposed to separate instead
of keeping together. They have no attraction for each other; but the
ball which the gas drives out of the gun leaves the gas behind it, and
goes on whole, because its particles are so well fastened together by
attraction.
You see, then, that in a solid there is considerable attraction between
the particles; in a fluid there is much less; and in a gas there is
none at all.
_Questions._--How does the pressure of a fluid differ from that of
a solid? Give the comparison about shot. Relate the experiment with
shot. Tell about the pile of cannon balls. Give the comparison about
shot and water running for an opening in a vessel. Why does water
run faster from an opening near the bottom of a vessel than from an
opening near the top? Why does it run more slowly as the water in the
vessel lessens? Give the comparison about a crowd going through a
door. Why does water run out from an opening in the side of a vessel
close to the bottom as fast as from a hole in the bottom itself? What
is the difference between pressing on a solid and pressing on a fluid?
How can you make a fluid all go one way in pressing it? What will
happen if there be a hole in the tube? Tell about the squirt-gun and
the stick-gun. Tell about the ball and the gas in a common gun. Tell
about attraction in solids, and fluids, and gases.