THE BAROMETER.
Water can be raised in a pump only to a certain height, and the mistake
has sometimes been made of getting the pump so long that it would not
work. If it be more than about thirty-four feet from the water up to
the piston, the water can not be made to go up so high. What is the
reason? It is because the air, pressing on the surface of the water in
the cistern or well, will raise it only to the height of thirty-four
feet. It does not press hard enough to force it up any higher.
Suppose you had a glass tube over thirty-four feet long, with one end
open, and used it as represented in the first experiment in Chapter
IV., on page 27. The water would be kept up in it only the thirty-four
feet. The weight of a column of water of that height just balances the
pressure or weight of the air. Above that height in the tube there
would be a space in which there would not be any thing.
[Sidenote: Pressure of the air holds up water in the pump and mercury
in the barometer.]
Quicksilver or mercury, as perhaps you know, is a fluid like water, but
very much heavier. The pressure of the air, therefore, will hold up a
column of this not nearly as high as the column of water it holds up.
The column of mercury held up in a glass tube is not quite three feet
long, while that of water is thirty-four feet.
[Illustration]
You can now understand how the instrument called a barometer is made.
The object of this is to tell how heavy the air is, for the air is
heavier at some times than it is at others. A glass tube, open at one
end, and about three feet in length, is taken, and is filled with the
mercury. Then the open end is put into a dish of mercury, as seen in
the figure. There will be a space in the tube above the mercury, as
represented, for the air will support by its pressure a column of only
about thirty inches of mercury--six inches less than three feet, the
length of the tube. A scale, divided into inches, is added, as seen in
the figure; and the whole, neatly inclosed in a case, makes what we
call a barometer. This means a measurer of the pressure or weight of
the air.
[Sidenote: Barometer on a mountain.]
If the barometer be carried up a mountain, the mercury falls. Why is
this? It is because there is less height of air pressing on the mercury
than there is in the valley below, and of course it will not hold up so
long a column of mercury. In the valley, as I have told you in Chapter
I., the air is forty-five miles high; and if we carry the barometer
up a mountain three or four miles high, it will make a difference of
several inches in the height of the mercury in the tube.
[Sidenote: Air heavier at some times than at others.]
I have said that the air is heavier at some times than at others. In a
bright, clear day, the air is heavy, and then the mercury rises high,
or, rather, is pushed up high in the tube. But when it is cloudy and
rainy, the mercury falls, for the air is then lighter than usual,
though people often say at such a time how heavy the air is. The truth
is that we feel better when the air is clear and heavy, and so the air
seems light to us. On the contrary, we do not feel so well when it is
cloudy and the air is light.
[Sidenote: How the barometer is of use to the sailor.]
The barometer is of use to the sailor in telling him of threatened
storms; for when a storm is coming the air is light, and the mercury
in the barometer falls of course. The sailor, therefore, looks now
and then at his barometer, and if he at any time sees the mercury
fall suddenly, he gets ready for a storm, for he knows that it may
come on very rapidly. Dr. Arnot says that he was once on board of a
vessel where the captain was enabled to save his ship and all on board
because he took warning in season from his barometer. The sun had just
set, and, as the evening was very pleasant, all on board were enjoying
themselves in various ways. But the captain’s orders were given to take
down sails and prepare for a storm. All were astonished, for nobody
could see any signs of a storm. But the captain had seen the mercury
sink down very suddenly in his barometer, and he knew that trouble was
coming, and probably very soon. He hurried the men, therefore, but the
storm came before he was quite ready. It was a violent hurricane. But
the ship, though much damaged, was saved, and in the morning the wind
was still, and all were rejoicing in their deliverance. Probably, if
the captain had not looked at his barometer, the ship, with all on
board, would have been lost.
_Questions._--How high can water be raised in a pump? Why can it not
be raised higher? Tell about the experiment with a long glass tube.
How high a column of mercury will the pressure of the air hold up?
Explain the barometer. Explain the falling of the mercury when the
barometer is carried up a mountain. How does the barometer show that
the air is heavier at some times than it is at others? Why does the
air seem light to us when it is heavy, and heavy when it is light? How
is the barometer of use to the sailor? Tell about the storm as related
by Dr. Arnot.