THE SEA AND THE ATMOSPHERE. 9 



enougli ; let ns see Avliat results arise from them : 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 

 absence from the polar regions must in like manner promote an 

 infinite accumulation of cold, to fit the entire earth for a ha- 

 bitation 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 description. 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 surflice to be supplied from the regions adjoining. Two 

 currents from the poles towai'd the equator are thus established 

 at the surface, while the sublimated air, diffusing itself by its 

 mobility, floAvs 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 refresh the torrid zone. 



29. Of diurnal rotation. — " Did the earth, as was long believed, 

 stand still while the sun circled around it, we should have liad 

 directly from north and south two sets of meridional currents 

 blowing at the surface of the earth toward the equator ; in the 

 upper regions we should have had them flowing back again to 

 the place whence they came. On the other hand, were the heat- 

 ing and cooling influences just referred to to cease, and the earth 

 to fail in impressing its own motion on the atmosphere, we should 

 have a furious hurricane rushing round the globe at the rate 

 of 1000 miles an hour — tornadoes of ten times the speed of the 

 most violent now known to us, sweeping everything before them. 

 A combination of the two influences, modified by the friction 

 of the earth, which tends to draw the air after it, gives us the 

 trade-winds, which, at the speed of from ten to twenty miles an 

 hour, sweep round the equatorial region of the globe un- 

 ceasingly. 



30. Currents. — " Impressed with the motion of the air, constantly 

 sweeping its surface in one direction, and obeying the same laws 

 of motion, the great sea itself would be excited into currents 

 similar to those of the air, were it not walled in by continents 

 and subjected to other control. As it is, there are constant 

 currents flowing from the torrid toward the frigid zone to supply 



