Q. _What is WIND?_

A. Wind is _air in motion_.

Q. _What PUTS the air in motion, so as to produce WIND?_

A. The princ.i.p.al causes are the _variations of heat and cold_, produced by the succession of _day and night_, and the _four seasons_.

Q. _What effect has HEAT upon the air?_

A. Heat _rarefies_ the air, and causes it to _expand_.

Q. _How do you KNOW that heat causes the air to EXPAND?_

A. If a bladder _half full of air_ (tied tight round the neck), were laid before a _fire_, the heat of the fire would expand the air so much, that the bladder would soon be _entirely inflated_; (in this case, the air in the bladder is expanded to _twice its original bulk_, by the heat of the fire).

Q. _What EFFECT is produced upon air by RAREFACTION?_

A. It causes the air to _ascend through colder strata_, as a cork (put at the bottom of a basin of water) would ascend through the water.

Q. _How do you KNOW that rarefied air ASCENDS?_

A. When a boy sets fire to the cotton of his balloon, the flame _heats the air_ inside the balloon; and the air becomes _so light_, that it ascends, and _carries the balloon with it_.

Q. _What effect is produced upon AIR by COLD?_

A. Air is _condensed by cold_, or squeezed into a smaller compa.s.s; in consequence of which, _it becomes heavier_, and descends towards the ground.

Q. _How do you KNOW that air is CONDENSED by COLD?_

A. After the bladder is _fully inflated_, (by lying before the fire), if it be taken _away from the fire_, the bladder will _collapse_, and show that it is not half full.

Q. _What is meant by the bladder "COLLAPSING?"_

A. The skin will become _wrinkled, shrivelled, and flabby_, because there is not sufficient air inside to _fill it out_.

Q. _How do you KNOW that CONDENSED air will DESCEND?_

A. As soon as the cotton of the balloon _is burnt out_, the air inside becomes _cold again_, and the balloon _falls to the earth_.

Q. _Does the SUN HEAT the AIR as it does the EARTH?_

A. No; the air is _not heated by the rays of the sun_, because air (like water) is a very _bad conductor_.

Q. _How is the AIR HEATED?_

A. By _convection_, thus:--The _sun_ heats the _earth_, and the _earth_ heats the _air resting upon it_; the air thus heated _rises_, and is succeeded by _other air_, which is heated in a similar way, till _all is warmed_ by "convective currents."

Q. _What is meant by "CONVECTIVE CURRENTS of air?"_

A. Streams of air heated by the earth, which _rise upwards_ and _carry heat with them_, are called "convective currents" of hot air.

Q. _Is the air in a ROOM in perpetual motion, as the air ABROAD is?_

A. Yes; there are always _two currents of air_ in the room we occupy, one of _hot_ air flowing _out_ of the room, and another of _colder_ air flowing _into_ the room.

Q. _How do you KNOW, that there are these TWO currents of air in every occupied ROOM?_

A. If I hold a lighted candle near the crevice _at the top of the door_, the flame will be blown _outward_ (towards the _hall_); but if I hold the candle _at the bottom of the door_, the flame will be blown _inwards_ (into the _room_).

Q. _Why would the flame be blown OUTWARDS (towards the HALL), if the candle were held at the TOP of the door?_

A. Because as the air of the room is warmed by the fire, &c., _it ascends_; and (floating about the upper part of the room) some of it escapes _through the crevice_ at the _top of the door_, and thus produces a current of air _outwards_ (into the _hall_).

Q. _Why would the flame be blown INWARDS (into the ROOM), if the candle were held at the BOTTOM of the door?_

A. Because after the warm air of the room has ascended to the ceiling, or made its escape into the hall, &c., a partial _vacuum_ is made at the _bottom of the room_; and cold air (from the hall) _rushes under the door_ to supply the void.

Q. _What is meant by a "partial VACUUM being made, at the BOTTOM of the ROOM?"_

A. A vacuum means a place _from which the air has been taken_: and a "_partial_ vacuum" means, a place from which a _part of its air_ has been taken away. Thus when the air on the floor _ascends to the ceiling_, a partial vacuum is made _on the floor_.

Q. _And how is the VACUUM filled UP again?_

A. It is filled up by _colder air_, which rushes (under the _door_, and through the _window_ crevices) into the room.

Q. _Give me an ILl.u.s.tRATION._

A. If I dip a pail into a pond and fill it with water, a hole (or vacuum) is made in the pond _as big as the pail_; but the moment I _draw the pail out_, the hole is _filled up_ by the water around.