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 ILLUSTRATION._ 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. Q. _Show how this illustration 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 brass meridian of a globe to represent the vertical rays of the sun; as you turn the globe round, _different parts_ of it will pass under the brass rim, in constant _succession_. 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._