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APPENDIX. 



and that heat is evolved ; but this is not the case with tin stone or 

 oxide of tin. 



In the preceding article, I have pointed out a probable cause of the 

 increase of temperature in the waters of the salt mines at Bex, in 

 Switzerland, which had not before been noticed. 



On the whole, however, making every allowance for errors from 

 various causes, the evidence for a considerable increase of heat with 

 the increase of depth in mines appears to be established, though the 

 amount of that increase remains to be ascertained. 



Humboldt states that, from observations made in mines and caves 

 in every zone, it is proved that the heat of the earth is much greater 

 than the mean temperature of the atmosphere at the same places. 



ON THE SURFACE OF THE M60N. 



Geologists have not hitherto regarded with due attention the phys- 

 ical structure of the moon : it is the only planetary body placed suffi- 

 ciently near us, to have the inequalities of its surface rendered dis- 

 tinctly visible with the telescope. Attendant on the earth, and hav- 

 ing the same quantity of solar light, and nearly the same density, we 

 may reasonably infer that the mineral substances of which it is com- 

 posed do not differ essentially from those on the surface of our own 

 planet. Astronomers now generally admit that the moon is surround- 

 ed with a very clear atmosphere ; but which is so low, that it scarcely 

 occasions a sensible refraction of the rays of light when it passes over 

 the fixed stars. Many of the dark parts of the moon, particularly the 

 part called Mare Crisium, appear to be covered with a fluid, which 

 may probably be more transparent and less dense than water, as the 

 form of the rocks and craters beneath it are seen, but not so distinctly 

 as in the lighter parts of the moon's surface. To examine the moon 

 with a reference to its external structure, the defining power of the 

 telescope should be of the first quality, sufficient to show the projec- 

 tions of the outer illuminated limb as distinctly as they appear when 

 the moon is passing over the disk of the sun, during a solar eclipse. 

 With such a telescope, and a sufficient degree of light and of magni** 

 fying power, almost every part of the moon's surface appears vol- 

 canic, containing craters of enormous magnitude and vast depth : the 

 shelving rocks, and the different internal ridges within them, mark the 

 stations at which the lava has stood and formed a floor during differ- I 

 ent eruptions ; while the volcanic cones in some of the craters resem- 

 ble those formed within the craters of modern volcanoes. 



The largest volcanic mountain on the southern limb of the moon, 

 (called by some astronomers Tycho, and by others Mount Sinai,) like 

 the largest volcanic mountain on the earth, Chimborazo, and like 

 Mont d'Or and the Puy de Dome in Auvergne, has no deep crater on 

 its summit. There are, indeed, the outlines of the crater, but it is 

 nearly filled up ; while, from the foot of this lunar mountain, diverg- 

 ing streams of lava flow in different directions, to the distance of six 

 hundred miles. The largest currents of lava, from lofty volcanoes on 



I 



