If a copper lid were used as a foot warmer, it would give the feet only .095 as much heat as an equal weight of water; a lead weight only .031 as much heat as water.  Flatirons are made of iron because of the relatively high specific heat of iron.  The flatiron heats slowly and cools slowly, and, because of its high specific heat, not only supplies the laundress with considerable heat, but eliminates for her the frequent changing of the flatiron.

18.  Water and Weather.  About four times as much heat is required to heat a given quantity of water one degree as to heat an equal quantity of earth.  In summer, when the rocks and the sand along the shore are burning hot, the ocean and lakes are pleasantly cool, although the amount of heat present in the water is as great as that present in the earth.  In winter, long after the rocks and sand have given out their heat and have become cold, the water continues to give out the vast store of heat accumulated during the summer.  This explains why lands situated on or near large bodies of water usually have less variation in temperature than inland regions.  In the summer the water cools the 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 for light and warmth.

The sun, therefore, is the great source of heat; whether it is the heat which comes directly from the sun and warms the atmosphere, or the heat which comes from burning coal, wood, and oil.