Inferences.—Heated gases rise because they expand. Hot-air balloons, winds, and heating with hot-air furnaces, all depend on this principle.
SOURCES OF HEAT AND LIGHT
NOTES FOR A SERIES OF LESSONS
1. The Sun.—Our dependence on it. Valuable results of its heat. Simple notions as to its size, distance, and nature. Our earth catches a very small fraction of the sun's heat; our sun is but one of millions—the fixed stars. Show the burning effect of a lens.
2. Fuels.—Wood, oil, coal, alcohol, gas, peat, straw: where obtained; special uses of each under varying conditions; need of economy. (This is closely related to geography.)
3. Electricity.—In urban schools use the electric light or some heating device for illustration. In rural schools a battery of two or three cells (see "Apparatus") will melt a fine strand drawn from a picture wire.
Applications: ironing, toasting, cooking; advantages or disadvantages compared with gas or wood.
4. Friction.—Pupils rub hands together; rub a button on a cloth; saw a string across the edge of a board or across the hand; bore a hole through a hardwood plank, then feel the auger-bit.
Applications: restoring circulation; "hot-boxes" in machinery; lubricants and their uses; lighting matches.
5. Pounding.—Hammer a nail flat on an anvil or stone; feel it. Bullets fired against an iron or stone surface may be picked up very hot. Note sparks that can be struck from a stone; percussion caps, flint-lock muskets.