On bare mountains, the heating and cooling effects of the sun are very
striking(Fig. 7); the surface of many a mountain peak is covered with
cracked rock so insecure that a touch or step will dislodge the
fragments and start them down the mountain slope. The lower levels of
mountains are frequently buried several feet under débris which has
been formed in this way from higher peaks, and which has slowly
accumulated at the lower levels.

5. Temperature. When an object feels hot to the touch, we say that
it has a high temperature; when it feels cold to the touch, that it
has a low temperature; but we are not accurate judges of heat. Ice
water seems comparatively warm after eating ice cream, and yet we know
that ice water is by no means warm. A room may seem warm to a person
who has been walking in the cold air, while it may feel decidedly cold
to some one who has come from a warmer room. If the hand is cold,
lukewarm water feels hot, but if the hand has been in very hot water
and is then transferred to lukewarm water, the latter will seem cold.
We see that the sensation or feeling of warmth is not an accurate
guide to the temperature of a substance; and yet until 1592, one
hundred years after the discovery of America, people relied solely
upon their sensations for the measurement of temperature. Very hot
substances cannot be touched without injury, and hence inconvenience
as well as the necessity for accuracy led to the invention of the
thermometer, an instrument whose operation depends upon the fact that
most substances expand when heated and contract when cooled.

[Illustration: FIG. 8.--Making a thermometer.]

6. The Thermometer. The modern thermometer consists of a glass tube
at the lower end of which is a bulb filled with mercury or colored