But why will it not remain more than 30 inches high in the tube?
You must remember it is only kept up in the tube at all by the
air which presses on the mercury in the cup. And that column of
mercury now balances the pressure of the air outside, and presses
down on the mercury in the cup at its mouth just as much as the
air does on the rest. So this cup and tube act exactly like a
pair of scales. The air outside is the thing to be weighed at
one end as it presses on the mercury, the column answers to the
leaden weight at the other end which tells you how heavy the air
is. Now if the bore of this tube is made an inch square, then
the 30 inches of mercury in it weigh exactly 15 lbs, and so we
know that the weight of the air is 15 lbs. upon every square
inch, but if the bore of the tube is only half a square inch, and
therefore the 30 inches of mercury only weigh 7 1/2 lbs. instead
of 15 lbs., the pressure of the atmosphere will also be halved,
because it will only act upon half a square inch of surface, and
for this reason it will make no difference to the height of the
mercury whether the tube be broad or narrow.
But now suppose the atmosphere grows lighter, as it does when it
has much damp in it. The barometer will show this at once,
because there will be less weight on the mercury in the cup,
therefore it will not keep the mercury pushed so high up in the
tube. In other words, the mercury in the tube will fall.
Let us suppose that one day the air is so much lighter that it
presses down only with a weight of 14 1/2 lbs. to the square inch
instead of 15 lbs. Then the mercury would fall to 29 inches,
because each inch is equal to the weight of half a pound. Now,
when the air is damp and very full of water-vapour it is much
lighter, and so when the barometer falls we expect rain.
Sometimes, however, other causes make the air light, and then,
although the barometer is low, no rain comes,
Again, if the air becomes heavier the mercury is pushed up above
30 to 31 inches, and in this way we are able to weigh the
invisible air-ocean all over the world, and tell when it grows
lighter or heavier. This then, is the secret of the barometer.
We cannot speak of the thermometer today, but I should like to
warn you in passing that it has nothing to do with the weight of
the air, but only with heat, and acts in quite a different way.
And now we have been so long hunting out, testing and weighing
our aerial ocean, that scarcely any time is left us to speak of
its movements or the pleasant breezes which it makes for us in
our country walks. Did you ever try to run races on a very windy
day? Ah! then you feel the air strongly enough; how it beats
against your face and chest, and blows down your throat so as to
take your breath away; and what hard work it is to struggle
against it! Stop for a moment and rest, and ask yourself, what
is the wind? Why does it blow sometimes one way and sometimes
another, and sometimes not at all?
Wind is nothing more than air moving across the surface of the
earth, which as it passes along bends the tops of the trees,
beats against the houses, pushes the ships along by their sails,
turns the windmill, carries off the smoke from cities, whistles
through the keyhole, and moans as it rushes down the valley.
What makes the air restless? why should it not lie still all
round the earth?
It is restless because, as you will remember, its atoms are kept
pressed together near the earth by the weight of the air above,
and they take every opportunity, when they can find more room, to
spread out violently and rush into the vacant space, and this
rush we call a wind.
Imagine a great number of active schoolboys all crowded into a
room till they can scarcely move their arms and legs for the
crush, and then suppose all at once a large door is opened. Will
they not all come tumbling out pell-mell, one over the other,
into the hall beyond, so that if you stood in their way you would
most likely be knocked down? Well, just this happens to the air-
atoms; when they find a space before them into which they can
rush, they come on helter-skelter, with such force that you have
great difficulty in standing against them, and catch hold of
something to support you for fear you should be blown down.
But how come they to find any empty space to receive them? To
answer this we must go back again to our little active invisible
fairies the sunbeams. When the sun-waves come pouring down upon
the earth they pass through the air almost without heating it.
But not so with the ground; there they pass down only a short
distance and then are thrown back again. And when these sun-
waves come quivering back they force the atoms of the air near
the earth apart and make it lighter; so that the air close to the
surface of the heated ground becomes less heavy than the air
above it, and rises just as a cork rises in water. You know that
hot air rises in the chimney; for if you put a piece of lighted
paper on the fire it is carried up by the draught of air, often
even before it can ignite. Now just as the hot air rises from
the fire, so it rises from the heated ground up into higher parts
of the atmosphere. and as it rises it leaves only thin air
behind it, and this cannot resist the strong cold air whose atoms
are struggling and trying to get free, and they rush in and fill
the space.
One of the simplest examples of wind is to be found at the
seaside. there in the daytime the land gets hot under the
sunshine, and heats the air, making it grow light and rise.
Meanwhile the sunshine on the water goes down deeper, and so does
not send back so many heat-waves into the air; consequently the
air on the top of the water is cooler and heavier, and it rushes
in from over the sea to fill up the space on the shore left by
the warm air as it rises. This is why the seaside is so pleasant
in hot weather. During the daytime a light sea-breeze nearly
always sets in from the sea to the land.
When night comes, however, then the land loses its heat very
quickly, because it has not stored it up and the land-air grows
cold; but the sea, which has been hoarding the sun-waves down in
its depths, now gives them up to the atmosphere above it, and the
sea-air becomes warm and rises. For this reason it is now the
turn of the cold air from the land to spread over the sea, and
you have a land-breeze blowing off the shore.
Again, the reason why there are such steady winds, called the
trade winds, blowing towards the equator, is that the sun is very
hot at the equator, and hot air is always rising there and making
room for colder air to rush in. We have not time to travel
farther with the moving air, though its journeys are extremely
interesting; but if, when you read about the trade and other
winds, you will always picture to yourselves warm air made light
by the heat rising up into space and cold air expanding and
rushing in to fill its place, I can promise you that you will not
find the study of aerial currents so dry as many people imagine
it to be.
We are now able to form some picture of our aerial ocean. We can
imagine the active atoms of oxygen floating in the sluggish
nitrogen, and being used up in every candle-flame, gas-jet and
fire, and in the breath of all living beings; and coming out
again tied fast to atoms of carbon and making carbonic acid.
Then we can turn to trees and plants, and see them tearing these
two apart again, holding the carbon fast and sending the
invisible atoms of oxygen bounding back again into the air, ready
to recommence work. We can picture all these air-atoms, whether
of oxygen or nitrogen, packed close together on the surface of
the earth, and lying gradually farther and farther apart, as they
have less weight above them, till they become so scattered that
we can only detect them as they rub against the flying meteors
which flash into light. We can feel this great weight of air
pressing the limpet on to the rock; and we can see it pressing up
the mercury in the barometer and so enabling us to measure its
weight. Lastly, every breath of wind that blows past us tells us
how this aerial ocean is always moving to and fro on the face of
the earth; and if we think for a moment how much bad air and bad
matter it must carry away, as it goes from crowded cities to be
purified in the country, we can see how, in even this one way
alone, it is a great blessing to us.
Yet even now we have not mentioned many of the beauties of our
atmosphere. It is the tiny particles floating in the air which
scatter the light of the sun so that it spreads over the whole
country and into shady places. The sun's rays always travel
straight forward; and in the moon, where there is no atmosphere,
there is no light anywhere except just where the rays fall. But
on our earth the sun-waves hit against the myriads of particles
in the air and glide off them into the corners of the room or the
recesses of a shady lane, and so we have light spread before us
wherever we walk in the daytime, instead of those deep black
shadows which we can see through a telescope on the face of the
moon.
Again, it is electricity playing in the air-atoms which gives us
the beautiful lightning and the grand aurora borealis, and even
the twinkling of the starts is produced entirely by minute
changes in the air. If it were not for our aerial ocean, the
stars would stare at us sternly, instead of smiling with the
pleasant twinkle-twinkle which we have all learned to love as
little children.
All these questions, however, we must leave for the present; only
I hope you will be eager to read about them wherever you can, and
open your eyes to learn their secrets. For the present we must
be content if we can even picture this wonderful ocean of gas
spread round our earth, and some of the work it does for us.
We said in the last lecture that without the sunbeams the earth
would be cold, dark, and frost-ridden. With sunbeams, but
without air, it would indeed have burning heat, side by side with
darkness and ice, but it could have no soft light. our planet
might look beautiful to others, as the moon does to us, but it
could have comparatively few beauties of its own. With the
sunbeams and the air, we see it has much to make it beautiful.
But a third worker is wanted before our planet can revel in
activity and life. This worker is water; and in the next lecture
we shall learn something of the beauty and the usefulness of the
"drops of water" on their travels.