A Guide to the Scientific Knowledge of Things Familiar
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 mass 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 space_ and is _less_ intense; just as a candle would _show less light_ in a _large_ room, than in a _small_ one.
Q. _Why is a mountain-top COLDER than a valley, "because the AIR there is LESS HEATED by REFLECTION?"_
A. Air is _not_ heated by the _sun_, but by _reflection from the surface of the earth_; and as there is _no earth_ round a _mountain-top_ to reflect heat, therefore the air there is intensely cold.
Q. _Why does RUBBING our HANDS and FACES make them feel WARM?_
A. Chiefly because the friction _excites the latent heat_ of our hands and faces, and makes it sensible to our feeling.
Q. _When a man has been almost DROWNED, why is suspended animation RESTORED by RUBBING?_
A. The vital heat of the body (which had become _latent_ by the action of the water) is _again developed by friction_: and, as soon as this animal heat can be excited, the vital powers of the body are restored.
Q. _Why do two pieces of ICE (rubbed together) MELT?_
A. Ice contains 140 _degrees of latent heat_, and (when two pieces are _rubbed together)_ their _particles are compressed_, and this _latent heat_ rolls out and _melts the ice_.
Q. _Are not FORESTS sometimes SET on FIRE by friction?_
A. Yes; when two branches or trunks of trees (blown about by the wind) _rub violently against each other_, their _latent heat is developed_, and sets fire to the forest.
Q. _What is meant by COMPRESSION?_
A. The act of _bringing parts nearer together_; as a sponge is _compressed_ by being _squeezed in the hand_.
Q. _Cannot HEAT be evolved from common air merely by COMPRESSION?_
A. Yes; if a piece of _German tinder_ be placed at the _bottom of a glass tube_, and the air in the tube _compressed by a piston_,[11] the tinder will catch fire.
[11] In a common syringe or squirt, the _handle_ part which _contains the sucker_ (and is forced up and down), is called "The Piston."
Q. _Why will the tinder catch fire?_
A. Because the _air is compressed_; and its _latent heat being squeezed out_, sets fire to the tinder at the bottom of the tube.