THE AERIAL OCEAN IN WHICH WE LIVE, 67 
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 to-day, 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 I 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 kep't pressed together near the earth by the 
F 2 
