294 LIGHT SCIENCE FOR LEISURE HOURS. 



manner the whole mass of gases which form the earth's 

 atmospheric envelope. The whole question of the 

 circulation of the air is investigated in Maury's in- 

 teresting work on the Physical Geography of the Sea, 

 and he appears to establish in the most convincing 

 manner the interchange of air between the northern 

 and southern hemispheres. 



And even if we could assume that the atmospheric 

 covering of any portion of the earth's surface was in 

 any way prevented from passing freely to other 

 regions, yet the cause assigned would be inadequate 

 to account for the difference of barometric pressure 

 actually existing between the two hemispheres. All 

 the land above the sea-level in the northern hemisphere, 

 if uniformly distributed over the surface of that 

 hemisphere, would be raised to a height of less than 

 200 feet above the present sea-level, and the actual 

 difference of level corresponding to the observed dif- 

 ference of barometric pressure is more than four times 

 as great. 



Passing over this theory as neither consistent with 

 the known laws regulating the motions of elastic fluids, 

 nor sufficient even if the consideration of those laws 

 were neglected, we come to the theory suggested by 

 Captain Maury a theory deserving of much more 

 attentive consideration. I shall quote his own words, 

 as the fairest method of presenting his theory; after 

 stating the observed difference of barometric pressure 

 in the two hemispheres, and mentioning the expulsion of 

 air from the northern hemisphere as the cause of this 



