TEMPERATURE OF THE EARTH IN TIME AND SPACE 7 



panied by extensive glaciation. Cooler climates have 

 occurred when the land areas were largest, or most 

 emergent, usually during the closing stages of periods or 

 eras, and cold climates nearly always existed during or 

 immediately following times of active mountain for- 

 mation. 



The two principal atmospheric factors in the regula- 

 tion of climate are carbon dioxide and water-vapor. The 

 latter is the more important of the two and is also the 

 more variable. If both of these atmospheric constituents 

 are present in great abundance, especially aqueous vapor, 

 a thickened atmospheric blanket will be formed which 

 will retard the heat loss of the earth and will also absorb 

 a greater amount of the sun's radiation. When there is 

 but little carbon dioxide gas and water-vapor, more heat 

 is lost and less absorbed. Climates then become cooler 

 and more variable. The amounts of carbon dioxide and 

 water-vapor in atmosphere have varied much during the 

 earth's history. 



Sources of Heat. — There are two sources of the heat 

 by means of which the earth surface may be warmed: 

 (1) radiant energy and (2) the internal heat of the 

 earth ^s mass. If either of these factors were to be re- 

 moved, life could no longer exist on the surface of the 

 earth. 



Temperature of the Earth's Interior. — There are 

 many reasons for believing that the interior of the earth 

 is very hot. Volcanic phenomena, geysers, and hot 

 springs show that the earth is very hot in certain locali- 

 ties, even near the surface. In going down into deep 

 mines the temperature rises a:bout one degree Fahren- 

 heit for each 100 feet. While there is little probability 

 that this rise in temperature which is usual near the 

 surface would continue at greater depths, yet it indicates 



