8 HOMOIOTHERMISM 



that at a depth of a few hundred miles, temperatures may 

 perhaps reach several thousand degrees. A slow dissemi- 

 nation of the internal heat toward the surface of the 

 earth continually takes place. 



Radiant Energy. — The chief source of radiant energy 

 is the sun. The amount of energy received from the 

 stars is not sufficient to produce a significant effect upon 

 the earth, but that received from the sun is very great. 

 It is measured by the heat produced when a given sur- 

 face exposed at right angles to a beam entirely absorbs 

 the radiant energy. The mean value of such determina- 

 tions is called *^the solar constant of radiation'' and 

 has been fixed by Abbot as 1.932 calories per square 

 centimeter per minute. If this value were constant the 

 earth would receive in a year something like one million 

 million million million calories. This would be enough 

 heat to melt a layer of ice 110 feet thick over the entire 

 surface of the earth, provided there was no reflection. 



The amount of radiant energy coming to the earth at 

 any given point depends upon several factors, among 

 which are longitude, distance between earth and sun, sun- 

 spots and other variations in the sun itself. The factor 

 which influences the heat converted from solar radiation 

 at any one point on the earth depends upon the intensity 

 of the solar radiation striking a given area, and the 

 absorbing capacity of the substance. A ** perfect black 

 body" is a highly efficient absorber of such radiations. 

 Obviously, surfaces that reflect radiations well have little 

 power to absorb. Snow is an excellent reflector and has 

 much to do with low temperatures during winter months. 

 Substances which are good transmitters of radiations 

 likewise absorb little of the radiant energy coming to 

 them. During volcanic eruptions the transmissibility of 

 the atmosphere undergoes marked changes, with the con- 



