8 THE PHYSICAL GEOGRAPHY OE THE SEA. 



The sea and the at- globe of lead sixtv milcs in diameter. Its principal 



mosphere contrast- , , . . i 



ed. elements are oxygen ana nitrogen gases, with a 



vast quantity of water suspended in them in the shape of vapor, 

 and commingled with these a quantity of carbon in the shape 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, dimin- 

 ished in weight, by heat ; like all fluids, they are mobile, tending 

 to extend themselves equally in all directions, and to fill up de- 

 pressions wherever vacant space will admit them ; hence, in these 

 respects, the resemblance betwixt their movements. Water is 

 not compressible or elastic, and it may be solidified into ice or vap- 

 orized into steam ; the air is elastic ; it may be condensed to any 

 extent by pressure, or expanded to an indefinite degree of tenuity 

 by pressure being removed from it ; it is not liable to undergo 

 any change in its constitution beyond these, by any of the ordi- 

 nary influences by which it is affected. 



28. " These facts are few and simple enough ; let us see what 

 Influence of the sun. Tcsults arisc from thcm : As the constant exposure 

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

 there engender a vast amount of heat, and as his absencev from 

 the polar regions must in like manner promote an infinite- accu- 

 mulation of cold, to fit the entire earth for a habitation to similar 

 races of beings, a constant interchange and communion betwixt 

 the heat of the one and the cold of the other must be carried on. 

 The ease and simplicity with which this is effected surpass all de- 

 scription. The air, heated near the equator by the overpowering 

 influences of the sun, is expanded and lightened ; it ascends into 

 upper space, leaving a partial vacuum at the surface to be sup- 

 plied from the regions adjoining. Two currents from the poles 

 toward the equator are thus established at the surface, while the 

 sublimated air, diffusing itself by its mobility, flows in the upper 

 regions of space from the equator toward the poles. Two vast 

 whirlpools are thus established, constantly carrying away the heat 

 from the torrid toward the icy regions, and, there becoming cold 

 'by contact with the ice, they carry back their gelid freight to re- 

 fresh the torrid zone. 



29. " Did the earth, as was long believed, stand still while the 

 Of diurnal rotation, sun circlcd arouud It, we should have had directly 



