Q. _Show how this ill.u.s.tration APPLIES._

A. The heated air which ascends from the bottom of a room, is as much _taken away_, as the water in the pail; and (as the void was instantly supplied by _other water in the pond_) so the _void of air is supplied_ by a current from _without_.

Q. _What is the CAUSE of WIND?_

A. The _sun_ heats the _earth_, and the _earth_ heats the _air resting upon it_; as the warm air ascends, the void is filled up by a _rush of cold air_ to the place, and this _rush of air_ we call WIND.

Q. _Does the WIND ALWAYS blow?_

A. Yes; there is always _some_ motion in the air; but the _violence_ of the motion is perpetually varying.

Q. _Why is there always SOME motion in the air?_

A. As the earth is _always turning round_, the vertical rays of the sun are always _varying_.

Q. _What do you mean by "the VERTICAL RAYS of the SUN?"_

A. The rays made at _noon-day_: when the sun is in a _direct line_ above any place, his rays are said to be "vertical" to that place.

Q. _How are the VERTICAL rays of the sun always VARYING?_

A. Suppose the bra.s.s meridian of a globe to represent the vertical rays of the sun; as you turn the globe round, _different parts_ of it will pa.s.s under the bra.s.s rim, in constant _succession_.

Q. _And is it NOON-DAY to the place over which the SUN is VERTICAL?_

A. Yes; as each place pa.s.ses _under the bra.s.s 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 bra.s.s 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 pa.s.sed the meridian_ is _cooling_: the air under _the vertical sun_ is the _hottest_; and the air which is over the place _about to pa.s.s 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_.

Q. _How can HILLS and MOUNTAINS ALTER the course of the WIND?_

A. Suppose a wind, blowing from the north, comes to a mountain, as it cannot pa.s.s _through it_, it must either rush _back again_, or _fly off at one side_ (as a _marble_ when it strikes against a _wall_).

Q. _Do MOUNTAINS affect the wind in any OTHER way?_

A. Yes; many mountains are _capped with snow_, and the _warm air_ is _condensed_ as it comes in contact with them; but as soon as the _temperature of the wind_ is changed, its _direction_ may be changed also.

Suppose A B C to be _three columns of air_. A, the column of air which is _cooling down_; B, the column to which the _sun is vertical_; and C, the column which _is to be heated next_. In this case the _cold_ air of A will rush towards B C, because the air of B and C is _hotter_ than A.

But suppose now C to be a _snow-capped mountain_. As the hot air of B reaches C, it is _chilled_; and (being now _colder_ than the air _behind_) it rushes _back again_ towards A, instead of following the sun.

Q. _How can the OCEAN affect the direction of the WIND?_

A. When the ocean rolls beneath the _vertical sun_, the water is _not made so hot_ as the _land_; and (as another _change of temperature_ is produced) another obstacle is offered to the _uniform direction of the wind_.

Q. _Why is not the WATER of the sea made so HOT by the vertical sun, as the surface of the LAND?_

A. 1st--Because the _evaporation_ of the sea is greater than that of the land:

2ndly--The waters are _never still_: and

3rdly--The rays of the sun strike _into_ the water, and are _not reflected from its surface_, as they are by _land_.

Q. _Why does the EVAPORATION of the sea prevent its surface from being HEATED by the vertical sun?_

A. As water _absorbs_ heat by being _converted into vapour_; the surface of the sea is continually _losing heat by evaporation_.

Q. _How does the MOTION of the sea prevent its surface from being HEATED by the vertical sun?_

A. As one portion is heated _it rolls away_, and is succeeded by _another_; and this constant motion prevents _one_ part of the sea from being heated _more than another_.

Q. _How is the WIND affected by the SEA?_

A. When air from the hot earth _reaches the sea_, it is often _condensed_, and either rushes _back again_, or else its violence is very greatly _abated_.

Q. _Do CLOUDS affect the WIND?_