that is next to the stove, for instance, becomes hot; this
hot air is pushed up and away by cold air, and carries its heat with
it. When it comes over to you in another part of the room, some of its
heat is conducted to your body. When air currents--or water currents,
which work the same way--carry heat from one place to another like
this, we say that the heat has traveled by _convection_.

[Illustration: FIG. 58. Convection currents carrying the heat of the
stove about the room.]

Since heat is so often carried to us by convection,--by warm winds,
warm air from the stove, warm ocean currents, etc.,--it _seems_ as if
air must be a good conductor of heat. But if you shut the air up into
many tiny compartments, as a bird's feathers do, or as the hair on an
animal's back does, so that it cannot circulate, the passage of heat
is almost completely stopped. When you use a towel or napkin to lift
something hot, it is not so much the fibers of cotton which keep the
heat from your hand; it is principally the very small pockets of air
between the threads and even between the fibers of the threads.

COLD THE ABSENCE OF HEAT. Cold is merely the absence of heat; so if
you keep the heat from escaping from anything warm, it cannot become
cold; while if you keep the heat from reaching a cold thing it cannot
become warm. A blanket is just as good for keeping ice from melting,
by shutting the heat out, as it is for keeping you warm, by holding
heat in.

_APPLICATION 32._ Explain why ice is packed in straw or
sawdust; why a sweater keeps you warm.

Select from the following list the good conductors of heat
from the poor conductors (insulators): glass, silver, iron,
wood, straw, excelsior, copper, asbestos, steel, nickel,
cloth, leather.

[Illustration: FIG. 59. Diagram of a hot-water heater. What makes the
water circulate?]

INFERENCE EXERCISE

Explain the following:

171. If the axle of a wheel is not greased, it swells until it
sticks fast in the hub; this is a hot box.

172. When you have put liquid shoe polish on your shoes, your
feet become cold as it dries.

173. The part of an ice-cream freezer which holds the cream
is usually made of metal, while that which goes outside and
contains the ice and salt is usually made of wood.

174. The steam in a steam radiator rises from a boiler in the
basement to the upper floors.

175. When you throw a ball,