6. Pressure.—After using a bicycle pump for some time, feel the bottom, also the top. If possible, examine an air-compressor and find out the means used for cooling the air.
7. Sources of Light.—Sun, moon, oil, tallow, gas, electricity, wax, acetylene; advantages of each; relative cost.
Primitive Methods of Obtaining Fire: Most savages obtain fire by friction; rubbing two pieces of wood together till hot enough to set fire to some dry, light material. The natives of Australia placed a flat piece of wood on the ground and pressed against this the end of a round piece, which they twirled rapidly with their hands till fire was produced. The North American Indians did the twirling with their bow strings; the Eskimo's plan is somewhat similar. It is impossible to say when flint and steel were first used, but we know they continued to be the chief means of producing fire till about 1834, when matches were invented. Let pupils try to produce fire by these means.
The earliest lamps consisted of shells, skulls of animals, and cup-shaped stones filled with fat or fish oils which burned on a wick of cloth or the pith of rushes. The Tibetans burn butter, the Eskimos whale- or seal-oil, the Arabians palm- or olive-oil. For outdoor lighting, torches carried in the hand were used till gas came into general use about 1792.
CONDUCTION
Give to four boys strips of copper, aluminium, wood, and glass, respectively. They hold these by one end and heat the other end till one or more are forced to drop the piece on account of the heat. The boys with the metals will soon find them hot throughout, but the other two will be able to hold on indefinitely. The teacher gives the terms "good conductor" and "poor conductor".
PROBLEMS
1. Are metals generally good conductors? Try with strips of zinc, lead, iron, a silver spoon.
2. Are all good conductors equally good? Devise a means of ascertaining. See Science of Common Life, Chapter VI; also The Ontario High School Physics, page 274.
3. Is water a good conductor?