

MEAN TEMPERATURE OF THE EARTH. 167 



at a depth of nearly 21 geographical miles, or between four and 

 five times the elevation of the highest summit of the Himalaya. 



We must distinguish in our globe three different modes for 

 the transmission of heat. The first is periodic, and affects the 

 temperature of the terrestrial strata according as the heat 

 penetrates from above downwards, or from below upwards, 

 being influenced by the different positions of the Sun and the 

 seasons of the year. The second is likewise an effect of the 

 Sun, although extremely slow : a portion of the heat that has 

 penetrated into the equatorial regions moves in the interior of 

 the globe towards the poles, where it escapes into the atmo- 

 sphere and the remoter regions of space. The third mode of 

 transmission is the slowest of all, and is derived from the secular 

 cooling of the globe, and from the small portion of the primi- 

 tive heat which is still being disengaged from the surface. 

 This loss experienced by the central heat must have been very 

 considerable in the earliest epochs of the Earth's revolutions, 

 but within historical periods it has hardly been appreciable by 

 our instruments. The surface of the Earth is therefore situated 

 between the glowing heat of the inferior strata and the uni- 

 versal regions of space, whose temperature is probably below 

 the freezing-point of mercury. 



The periodic changes of temperature which have been 

 occasioned on the Earth's surface by the Sun's position and 

 by meteorological processes, are continued in its interior, 

 .although to a very inconsiderable depth. The slow con- 

 ducting power of the ground diminishes this loss of heat 

 in the winter, and is very favourable to deep-rooted trees. 

 Points that lie at very different depths on the same vertical 

 line attain the maximum and minimum of the imparted 

 temperature at very different periods of time. The further 

 they are removed from the surface the smaller is this dif- 

 ference between the extremes. In the latitudes of our tem- 

 perate zone (between 48 and 52) the stratum of invariable 

 temperature is at a depth of from 59 to 64 feet, and at half 

 that depth the oscillations of the thermometer, from the 



.as I have already remarked, excavations are going on at a depth of about 

 1500 feet below the level of the sea, an increase of 1 to 59*06 feet, 

 a result almost identical with that found by Arago in the Puits de 

 Grenell. 



