164 TERRESTRIAL 



or, inversely, escapes by the same path. The second is 

 also an effect of the Sun; its action is extremely slow, 

 part of the heat which has penetrated into the Earth in the 

 equatorial regions travels along the interior of the Earth's 

 crust to the vicinity of the poles, where it escapes into the 

 atmosphere, and thence into space. The third is the slowest 

 of all ; it consists in the secular cooling of the terrestrial 

 globe ; in the escape of the very small quantity of the primi- 

 tive heat of the planet which is now given out from its surface. 

 The loss of central heat is supposed to have been very great at 

 the time of the early terrestrial revolutions, but within his- 

 toric periods it has hardly been appreciable by our instru- 

 ments. The temperature of the surface of the earth is 

 intermediate between the glowing temperature of the 

 inferior strata, and that of space which is probably below 

 the freezing point of mercury. 



The periodic variations of the temperature, produced at 

 the surface by the position of the Sun and by meteorolo- 

 gical processes, propagate themselves towards the interior 

 of the Earth, but only to a very inconsiderable depth. The 

 slow conducting power of the soil diminishes the loss of 

 heat in winter, and is favourable to trees having deep roots. 

 At points placed at different depths on the same vertical 

 line, the maximum and minimum of the imparted tempera- 

 tures are attained at very different seasons ; and the greater 

 the distance from the surface, the less is the difference be- 

 tween the extremes of temperature. In our temperate 

 latitudes (48 52) the stratum of invariable temperature 

 is found at a depth of from 55 to 60 French feet (59 to 64 

 English feet nearly) ; and at half that depth the oscillations 

 of temperature from the influence of season are already 



