region; in the winter, on the contrary, the water heats the region,
and hence extremes of temperature are practically unknown.

19. Sources of Heat. Most of the heat which we enjoy and use we owe
to the sun. The wood which blazes on the hearth, the coal which glows
in the furnace, and the oil which burns in the stove owe their
existence to the sun.

Without the warmth of the sun seeds could not sprout and develop into
the mighty trees which yield firewood. Even coal, which lies buried
thousands of feet below the earth's surface, owes its existence in
part to the sun. Coal is simply buried vegetation,--vegetation which
sprouted and grew under the influence of the sun's warm rays. Ages ago
trees and bushes grew "thick and fast," and the ground was always
covered with a deep layer of decaying vegetable matter. In time some
of this vast supply sank into the moist soil and became covered with
mud. Then rock formed, and the rock pressed down upon the sunken
vegetation. The constant pressure, the moisture in the ground, and
heat affected the underground vegetable mass, and slowly changed it
into coal.

The buried forest and thickets were not all changed into coal. Some
were changed into oil and gas. Decaying animal matter was often mixed
with the vegetable mass. When the mingled animal and vegetable matter
sank into moist earth and came under the influence of pressure, it was
slowly changed into oil and gas.

The heat of our bodies comes from the foods which we eat. Fruits,
grain, etc., could not grow without the warmth and the light of the
sun. The animals which supply our meats likewise depend upon the sun