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