CHAPTER VIII.. 2.--Friction.

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, the tinder will catch fire.

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.