Q. And is it noon-day to the place over which the sun is vertical?
A. Yes; as each place passes under the brass meridian, it is noon-day to one half, and mid-night to the other.
Q. Show how this rotation of the earth affects the air.
A. If we suppose the brass meridian to be the vertical sun, the whole column of air beneath will be heated by the noon-day rays; that part which the sun has left, will become gradually colder and colder; and that part to which the sun is approaching, will grow constantly warmer and warmer.
Q. Then there are three qualities of air about this spot?
A. Yes; the air over the place which has passed the meridian is cooling: the air under the vertical sun is the hottest; and the air which is over the place about to pass under the meridian, is increasing in heat.
Q. How does this variety in the heat of air produce wind?
A. The air always seeks to preserve an equilibrium; so the cold air rushes to the void, made by the upward current of the warmer air.
Q. Why does not the wind always blow one way, following the direction of the sun?
A. Because the direction of the wind is subject to perpetual interruptions from hills and valleys, deserts and seas.