Hot and Thermal Springs. 151 



and if we give to a, n, and ni, the same values as in the above 



example, we shall find 



p = 0°.0267 

 B K = 24.7 feet 



Under these circumstances, then, the increase of temperature 

 towards the centre of the earth, would be found the same at K, 

 at a distance of 247 feet from the point B, on the plateau B M, 

 as in plains. 



If the plateau B M be not horizontal, the value of B K may 

 be easily deduced from the above formula. If B M rise, for 

 example, at an angle of b from the horizon, the angle a is 

 changed into a — b. If, on the contrary, B M fall at an angle 

 of b, the angle a is changed into a + b. In the equation of 

 h K, therefore, instead of a we must put, in the former case 

 a — b, in the latter a -{- b. 



The formula; calculated above, enable us to describe the 

 curves of the chthonisothermal lines in all mountains. It is 

 evident, that they must form very irregular curves in moun- 

 tains of irregular shapes. If a B C/' (Pi. II. Fig. 10.) represent 

 the vertical section of a mountain, the sides of which have an 

 inclination of 30° and 72% and having a horizontal plateau on 

 its summit; and if, further, its height be 677 feet, so thai, as it 

 is above supposed, the temperature decrease 2°.25 from its foot 

 to its summit, then the chthonisothermal line having the mean 

 temperature of the foot of the mountain, will take the direction 

 of the lines a i c (Z cyr The numbers 127, 115, and 249, re- 

 present the depths in feet, in which the temperature increases 

 S'.SS, and 25 and 69 feet are the distances from B and C, at 

 which the normal increase of temperature of 2°.25 in 115 feet 

 will begin to prevail on the [)lateau. The dimensions in the 

 figure are drawn exactly according to a scale. 



Among all the observations which have hitherto been made 

 for the sake of discovering, in a direct manner, the increase of 

 temperature towards the interior of the earth, I only know one 

 which has been made in an almost horizontal direction. It 

 is that one which was made by Boussingault* in the metal- 

 liferous mountains of Marmato. This mountain is so steep 



* Aniial. lie ("him. and do Tlivs., vol. liii., ji. 210, 



