CHAPTER VII.. Mechanical Action.

MECHANICAL ACTION.

1.--PERCUSSION.

Q. How is heat produced by MECHANICAL ACTION?

A. 1.--By Percussion. 2.--By Friction. 3.--By Condensation.

Q. What is meant by PERCUSSION?

A. The act of striking; as when a blacksmith strikes a piece of iron on his anvil with his hammer.

Q. Why does BEATING IRON make it RED-HOT?

A. Beating the iron condenses the particles of the metal; and squeezes out its latent heat, as water from a sponge.

Q. Does COLD iron contain HEAT?

A. Yes; every thing contains heat; but when a thing feels cold, its heat is LATENT.

Q. What is meant by LATENT HEAT?

A. Heat not perceptible to our feeling. When anything contains heat without feeling the hotter for it, that heat is called "latent." (See p. 31.)

Q. Does COLD iron contain latent HEAT?

A. Yes; and when a blacksmith compresses the particles of the iron by his hammer, he squeezes out this latent heat, and makes the iron red-hot.

Q. How did blacksmiths use to LIGHT THEIR MATCHES before the general use of lucifers?

A. They used to place a soft iron nail upon their anvil; strike it two or three times with a hammer; and the point became sufficiently hot to light a brimstone match.

Q. How can a NAIL (beaten by a hammer) IGNITE a brimstone MATCH?

A. As the particles of the nail are compressed by the hammer, it cannot contain so much heat as it did before; so some of it flies out (as water flows from a sponge when it is squeezed).

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.