Q. _Why does STRIKING a FLINT against a piece of STEEL produce a SPARK?_

A. The blow _condenses_ those parts of the flint and steel which strike _together_, and squeezes out their latent heat.

Q. _How does this development of HEAT produce a SPARK?_

A. A very small fragment (either of the steel or flint) _is knocked off red-hot_, and sets fire to the tinder on which it falls.

Q. _Why is it needful to keep BLOWING the TINDER with the breath?_

A. Because _blowing_ the tinder, drives the _oxygen of the air_ towards it.

Q. _Where does the OXYGEN of the air COME FROM, which is blown to the lighted tinder?_

A. The air itself is composed of two gases (_nitrogen and oxygen_) mixed together.

(Every 5 lbs. of common air contain 4 lbs. of nitrogen, and 1 lb. of oxygen.)

Q. _What is the good of BLOWING OXYGEN GAS to lighted tinder?_

A. Oxygen gas _supports combustion_; and lighted tinder is _quickened by the breath_, in the same way as a dull fire is revived by a _pair of bellows_.

Q. _Why do HORSES sometimes STRIKE FIRE with their FEET?_

A. When iron horse-shoes strike against the flint-stones of the road, _very small fragments_ (either of the shoe or stones) are _knocked off red-hot_, and look like sparks.

Q. _What makes these fragments RED-HOT?_

A. The percussion _condenses_ the part struck, _and squeezes out its latent heat_.

CHAPTER VIII.

2.--FRICTION.

3.--CONDENSATION.

Q. _What is meant by FRICTION?_

A. The act of _rubbing two things together_; as the Indians rub two pieces of _wood_ together to produce fire.

Q. _How do the Indians produce FIRE, by merely RUBBING TWO PIECES of dry WOOD TOGETHER?_

A. They take a piece of dry wood (sharpened to a point), which they rub quickly up and down a _flat piece_, till a _groove_ is made; and the _saw-dust_ (collected in this groove) soon _catches fire_.

Q. _Why does the saw-dust of the WOOD CATCH FIRE by RUBBING?_

A. The _latent heat_ of the wood is _developed by friction_; because the particles of the wood are _squeezed closer together_, and the heat pours out, as water from a sponge.

(The best woods for this purpose are _box-wood_ against _mulberry_, or _laurel_ against _poplar_ or _ivy_.)

Q. _Do not CARRIAGE WHEELS sometimes CATCH FIRE?_

A. Yes; if the wheels be _dry_,--or _fit too tightly_,--or _revolve very rapidly_,--they often catch fire.

Q. _Why do wheels catch fire in such cases?_

A. The _friction_ of the wheels against _the axle-tree_ is so great, that their _latent heat is disturbed_, and produces ignition.

Q. _What is the use of GREASING CART WHEELS?_

A. The grease _lessens the friction_; and (by diminishing the _friction_) the latent heat is less disturbed.

Q. _Why is the TOP of a MOUNTAIN COLDER than the VALLEY beneath, although it be two or three miles nearer to the sun?_

A. 1st--Because the air on a mountain is _less compressed_, than the air in a valley.

2ndly--It is _more rarefied_: and

3rdly--It is _less heated by reflection_.

Q. _Why is air COLDER on a mountain "because it is LESS COMPRESSED?"_

A. As the air in a _valley_ is more compressed (by the ma.s.s of air above) than that on the top of a _mountain_, therefore _more heat runs out_; just as more water runs from a sponge, the closer it is _squeezed together_.

Q. _Why is a mountain-top COLDER than a valley, "because the AIR there is MORE RAREFIED?"_

A. As the air is _more rarefied_, its heat is _diffused over a larger s.p.a.ce_ and is _less_ intense; just as a candle would _show less light_ in a _large_ room, than in a _small_ one.