Q. _Why does not WOOD MELT, like metal?_ A. Because the heat of the fire _decomposes_ the wood into _gas, smoke, and ashes_; and the different parts _separate from each other_. Q. _What is meant by VAPORIZATION?_ A. The _conversion of liquid into vapour_; as water is converted into vapour by the heat of the sun. Q. _What are CLOUDS?_ A. Moisture _evaporated from the earth_, and collected in the upper regions of the air. Q. _What is the difference between a FOG and a CLOUD?_ A. Clouds and fogs differ only in one respect. _Clouds are elevated above our heads_: but _fogs come in contact with the surface of the earth_. Q. _If CLOUDS are WATER, why do they FLOAT on the air?_ A. 1st--The vapour of clouds is composed of _very minute bubbles_ (called ves'cicles), which float like _soap bubbles_: and 2ndly--Warm air (between the bubbles) _keeps them apart_, and makes the mass _lighter_; and the currents of air (which constantly ascend from the warm earth) _buoy them up_. Q. _Why does VAPOUR sometimes form into CLOUDS, and sometimes rest upon the earth as MIST or FOG?_ A. When the _surface of the earth_ is _warmer than the air_, the vapour of the earth (being condensed by the chill air) becomes _mist or fog_. But when the _air_ is _warmer than the earth_, the vapour _rises through the air_, and becomes cloud. Q. _Are ALL clouds ALIKE?_ A. No. They vary greatly in _density, height, and colour_. Q. _What is the chief CAUSE of fog and CLOUDS?_ A. The changes of the wind. Q. _How can the CHANGES of the WIND affect the CLOUDS?_ A. If a _cold current of wind_ blows suddenly over any region, it _condenses_ the invisible vapour of the air into _cloud or rain_: but if a _warm current of wind_, blows over any region, it _disperses_ the clouds, by _absorbing their vapour_. Q. _What_ COUNTRIES _are the_ MOST CLOUDY? A. Those where the winds are _most variable_, as Britain. Q. _What COUNTRIES are the LEAST cloudy?_ A. Those where the winds are _not variable_, as Egypt. Q. _What DISTANCE are the CLOUDS from the EARTH?_ A. Some _thin light clouds_ are elevated above the highest mountain-top; some _heavy_ ones touch the steeples, trees, and even the earth: but the _average_ height is between _one and two miles_. (Streaky curling clouds, _like hair_, are often five or six miles high.) Q. _What CLOUDS are the LOWEST?_ A. Those that are _most highly electrified_: lightning clouds are rarely more than about 700 yards above the ground; and very often actually _touch the earth with one of their edges_. Q. _What is the THICKNESS of the CLOUDS?_ A. Some clouds are 20 _square miles in surface_, and above _a mile in thickness_; while others are only a _few yards or inches_. Q. _How can persons ascertain the thickness of a cloud?_