Xll CONTENTS. 



LETTER VI. 



Page 

 JETS OP GAS, AND MUD-VOLCANOES ... ... 74 



LETTER VII. 



VOLCANOES AND EARTHQUAKES ... ... 87 



LETTER VIII. 



THE TEMPERATURE OP THE OUTERMOST CRUST OP 



THE EARTH ... ... ... ... 115 



LETTER IX. 



THE TEMPERATURE OE THE LOWEST STRATUM OF 



THE ATMOSPHERE ... ... ... 132 



LETTER X. 



THE TEMPERATURE OP THE HIGHER LAYERS OP 



THE AIR ... ... ... ... 148 



LETTER XL 



THE TEMPERATURE OP THE WATERS, AND THEIR 



INFLUENCE ON CLIMATE ... ... ... 168 



LETTER XII. 



THE CURRENTS OP THE SEA ... ... ... 194 



