152 Prof. Bisdioff" on the Temperature of 



that it may almost be called an enormous wall of porphyritic 

 syenite. Several adits enter it at different heights. At the 

 mouth of one of these adits, 4494 feet above the level of the sea, 

 the mean temperature is 68°.0, and the temperature increases to- 

 wards the interior of the mountain at the rate of 2°.25 in about 

 127.5 feet. Boussingault says about, because it is not uniform, 

 but varies according to the thickness of the rock above the adit, 

 or, which is the same thing, according to the form of the surface 

 of the mountain. The miners work in an atmosphere, the tem- 

 perature of which is nearly equal to that on the sea-coast. 



This fact, together with the remark, that the increase of tem- 

 perature is regulated by the thickness of the rock above the 

 adit, proves very clearly that the increase of temperature can 

 only be measured perpendicularly to the surface of the earth. 

 It is singular that the above result accords within a foot with 

 the increase of temperature found in a vertical direction, in the 

 Erzgebirge (see Chap. XIX). Supposing the side of the moun- 

 tains of Marmato to be quite perpendicular, and that the tem- 

 perature of the surface there decreases 2°.25 in 690 feet addi- 

 tional elevation (see Chap. XVI II.), the chthonisothermal line, 

 having the mean temperature of the foot of the mountain, would 

 then rise at an angle of 79°.32 with the horizon. This is, in 

 general, about the maximum angle at which chthonisothermal 

 lines can rise in mountains having a perpendicular ascent. 



We must not leave the question undecided, whether the va- 

 lue of n, taken in a direction perpendicular to the surface of the 

 earth, is always the same ; all other circumstances remaining the 

 same, whether in plains, in deep valleys, surrounded with high 

 mountains, or in mountains. If the nucleus of the earth pos- 

 sesses a certain quantity of heat; which is radiated towards all 

 points of its surface, it might be expected that this distribution 

 of the heat would be unequal at different points, according as 

 they were situated on lofty projections or in deep hollows ; so 

 that in the mountains, at the greatest distance from the centre 

 of heat, the increase of temperature would proceed more slowly 

 than under plains and valleys. But however probable it may 

 be, that a certain portion of the heat which spreads to the more 

 distant points, suffers a slower decrease from the centre towards 

 the surface ; yet it must, on the other hand, not be forgotte 



