32 INTERIOR CONSTITUTION OF THE EARTH 



extreme than in the air. Penetrating still deeper, we come to a 

 level where the temperature remains the same throughout the 

 year, and is also the same as the annual average temperature of 

 the air at the same locality above ground. This shows that the 

 temperature at this level of no variation is determined by the solar 

 heat and other climatic factors. The depth at which this level is 

 situated depends upon the latitude of the place where the obser- 

 vation is made. At the equator it is only three or four feet below 

 the surface of the ground, while in the polar regions it is said to 

 be several hundred feet below. This does not imply that the 

 effects of the sun's heat penetrate less deeply at the equator than 

 at the poles, but that the small variations of atmospheric tempera- 

 tures "in the tropics are equalized at shallow depths within the 

 earth. In temperate regions this level occupies an intermediate 

 position, and at the latitude of New York is found at a depth of 

 about fifty feet. 



Beneath the level of no variation the temperature increases with 

 the depth, though at very different rates in different localities. In- 

 creasing heat with increasing depth is observed in all deep mines, 

 tunnels, and borings, and in some cases the high temperature is a 

 very effective barrier against any further penetration of deep shafts. 

 In locating the great tunnels under the Alps the engineers have 

 been compelled to exercise great care, lest temperatures exceed- 

 ing those in which men can work should be encountered. The 

 average rate of increase of temperature is usually put at i° Fahren- 

 heit for every 55 to 60 feet of descent, but recent observations made 

 in the very deep shafts of the Calumet mine in the Lake Superior 

 copper region, indicate that this rate is perhaps too high. Should 

 this rate be continued regularly, it would give at a depth of 25 to 

 30 miles a heat sufficient to melt almost any rock, at atmospheric 

 pressw-es. 



Physical State of the Earth's Interior. — Astronomers and 

 geologists agree in the opinion that the earth, at one stage- in its 

 history, was a nebulous mass, and that it has reached its present 

 state by cooling and contraction. When, however, we attempt 

 to determine how far solidification has proceeded, and what the 



