Now when we wish to see the weight of the air we consult
a barometer, which works really just in the same way
as the water in this tube. An ordinary upright
barometer is simply a straight tube of glass filled
with mercury or quicksilver, and turned upside-down
in a small cup of mercury. The tube is a little
more than 30 inches long, and though it is quite full
of mercury before it is turned up, yet directly it
stands in the cup the mercury falls, till there is
a height of about 30 inches between the surface of
the mercury in the cup, and that of the mercury in
the tube. As it falls it leaves an empty space
above the mercury which is called a vacuum, because
it has no air in it. Now, the mercury is under
the same conditions as the water was in the U tube,
there is no pressure upon it at the top of the tube,
while there is a pressure of 15 lbs. upon it in the
bowl, and therefore it remains held up in the tube.
Week 9
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.
