Part 22 (2/2)
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 diminis.h.i.+ng 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.
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