136 SCIENCE OF COMMON THINGS. 



Heat of the earth. Cause of earthquakes. 



CHAPTEE III. 



OTHER SOURCES OF HEAT BESIDES THE SUN. 



862 How far below the surface of the earth does the influence of solar 

 heat extend ? 



The depth varies from 50 to 100 feet / never, how- 

 ever, exceeding the latter distance. 



863 How do we know that the earth is a source of heal f 



Because we find as we descend into the earth and 

 pass beyond the limit of solar heat, that the tempera- 

 ture constantly increases. 



864 At what rate does the temperature increase f 



About one degree of the thermometer for every fifty 

 feet. 



865 Supposing the temperature to continue to increase according to this 

 ratio, what would be its effects at different deptJis 1 



At the depth of two miles water would be converted 

 into steam ; at four miles, tin would be melted ; at 

 five miles, leaa; and at thirty miles, almost every 

 earthy substance would be reduced to a fluid state. 



866 To what cause may earthquakes and volcanoes be attributed f 



Undoubtedly to the agency of the internal haat of 

 the earth. 



867 What effect has the internal heat of the earth on the temperature 

 of the surface ? 



JVo sensible effect: it has been calculated that it 

 affects the temperature of the surface less than ^V of a 

 degree of the thermometer. 



868 WAy, if so great an amount of heat exist in the interior of the 

 earth, does it not appear more manifest upon the surface f 



Because the materials of which the exterior strata or 

 crust of the earth is composed, do not conduct it to the 

 surface from the interior. 



869 In what manner is electricity a source of heat? 



When an electric current passes from one substance 



