and
convection. We say this heat is _radiated_ from the iron. The velocity
of radiated heat is about 186,000 miles a second.

2. The above experiment may be varied by bringing the hot iron gradually
toward the bulb of the air thermometer and noting the greatest distance
at which it will affect the thermometer.

It is by radiation that the sun's heat and light reach us. We get much
of the heat of stoves, fire-places, and radiators by the same means.

Why does the earth cool off at night? Why does dew form? Why can no dew
form on a cloudy night? Why is a mountain top or a desert so cold,
especially at night?

3. Take two tin cans (baking powder boxes will answer) and make holes in
the lids large enough to admit a thermometer. Blacken one box in the
flame of an oil lamp. Fill both with boiling water and put in a cool
place. Test with a thermometer from time to time. Which cools most
rapidly?

4. Fill the tin cans with cold water, find the temperature, and then
place them near a hot stove. Which warms faster? Usually dark or rough
surfaces radiate heat and absorb heat faster than bright or smooth ones.
An excellent way of testing this is to lay a black cloth and a white one
side by side on the snow where the sun is shining brightly. The snow
will melt more rapidly under the black cloth. Painted shingles may be
substituted for the cloths. Try different colours. The day chosen should
not be extremely cold.

PROBLEMS

1. Why should we have the outside of a tea-kettle, teapot, or hot-air
shaft of a bright colour? Why should we have stoves and stove-pipes dull
black?

2. Why does a coat of snow keep the earth warm?

3. Which is the warmest colour to wear in winter? Does this account for
the colour of Arctic animals?

4. Which is the coolest colour to wear in the hot sun?

5. Gardeners sometimes strew the ground with coal-dust to help ripen
their melons. Show the value of this.

6. Suggest a method of protecting a wall from the heat of a stove.

CHAPTER XI

FORM III

SPRING

WINDOW BOXES

Many garden plants should be started in a box of earth in a warm, sunny
window. In some schools this can be done with a little care in heating
on cold nights. Small boxes or grape baskets full of rich sandy loam
with an inch of gravel in the bottom for drainage may be used. Sow the
seeds in rows or broadcast. To prevent the soil from drying out too
quickly, cover the box with a pane of glass. When the plants are