Physica I Geog raphy of Hindos tan . 341 



is somewhere about four miles — the greatest depth the 

 sounding line till of late has ever reached is five-and-a- 

 quarter miles. The mean elevation of the land again is 

 about one thousand feet — the highest point known to us 

 is nearly as much above the level of the sea, as the 

 greatest depth that has been measured is below it. The at- 

 mosphere again surrounds the earth like a vast envelope : 

 its depth, by reason of the tenuity attained by it as the super- 

 incumbent pressure is withdrawn, is unknown to us, — but 

 is guessed at somewhere betwixt fifty and five hundred miles, 

 its weight and its constituent elements have been determined 

 with the utmost accuracy. The weight of the mass is equal 

 to that of a solid globe of lead sixty miles in diameter. Its 

 principal elements are oxygen and nitrogen gases, with 

 a vast quantity of water suspended in these in the shape of 

 vapour ; and communicating with these a quantity of car- 

 bon, in the form of fixed air, equal to restore from its 

 mass many-fold the coal that now exists in the world. 

 In common with all substances, the ocean and the air 

 are increased in bulk, and consequently diminished in weight, 

 by heat; like all fluids, they are mobile — tending to 

 extend themselves equally in all directions, and to fill up 

 depressions in whatever vacant spaces will admit them ; 

 hence, in these respects, the resemblance betwixt their move- 

 ments. Water is not compressible or elastic, and it may be 

 solidified into ice, or vapourized into steam ; air is elastic — it 

 may be condensed to any extent by compression, or expanded 

 to an indefinite degree of tenuity by pressure being removed 

 from it — it is not liable to undergo any change in its consti- 

 tution beyond these, by any of the ordinary influences by which 

 it is affected. These facts are few and simple enough — let 

 us see what results arise from them. As the constant ex- 

 posure of the equatorial regions of the earth to the sun must 

 necessarily here engender a vast amount of heat, — and as 

 his absence from the polar regions must in like manner pro- 

 mote an infinite accumulation of cold,— to fit the entire earth 

 for a habitation to similar races of beings, a constant inter- 

 change and communion betwixt the heat of the one and 

 cold of the other must be carried on. The ease and simpli- 

 city with which this is effected, surpass all description. The 



