ISOTHERM OF 0°. 143 



contracts again to a breadth of about 23 degrees 

 latitude, which it retains till it reaches the eastern 

 coast of the new continent. 



While the mean temperature of 0° reaches in 

 Europe up to 71° N. L., and on the west coast of 

 America up to 66° N. L., in the interior of the 

 great continents, and on the eastern coasts, it 

 does not pass the limit of 55° N. L. The neigh- 

 bourhood of the sea has, therefore, an unmis- 

 takeable influence in moderating the temperature 

 on the western sea-borders, which it does not 

 exert on the eastern coasts. This action is 

 gradually weakened towards the interior of the 

 continent, and is strikingly diminished where the 

 high mountains, ranging north and south, separate 

 the coast lands from the interor. 



In all places on the earth through which the 

 isotherm of 0° passes, there cannot prevail, at the 

 underground limits of constant heat, a higher 

 temperature than is represented by as many 

 degrees above 0° as are due to the action of 

 terrestrial heat. If, for instance, you were to meet 

 at a depth of one hundred feet, with the limit I 

 have mentioned, you would probably find there 

 a constant temperature of about 1° C. (33°*8 Fahr.) 

 Farther northwards, where the mean temperature 

 of the ground falls below 0°, there must, there- 

 fore, occur a constant temperature of 0°, or even 

 below 0° (32° Fahr.), at such a depth beneath the 



