LOW BAROMETER OF ANTARCTIC ZONE. 293 



We may, however, reasonably inquire whether the 

 presence of continents in the northern hemisphere 

 might not operate in another manner. If we place 

 any mass within a vessel containing fluid, it is clear 

 that we increase the fluid pressure over every point 

 of the vessel's bottom, because this pressure depends 

 wholly on the depth of the bottom below the level of 

 the fluid, and the level rises when any solid substance 

 is placed within the vessel. Now if we suppose a globe 

 covered all over by water to be surrounded by a 

 perfectly uniform atmospheric envelope, the mean 

 pressure of this envelope at the water-level would 

 certainly be increased if continents were supposed to be 

 raised in any manner above the surface of the water ; 

 and if the atmosphere over one half of such a globe 

 were supposed to be prevented in any way from mixing 

 freely with the atmosphere over the other half, then it 

 is clear that the mean pressure at the water-level would 

 be greatest on that half-globe over which the most 

 extensive and highest continents had been raised. On 

 the assumption, then, of some such arrangement over 

 our own earth an arrangement, that is, which should 

 prevent the northern air from mixing with the southern 

 one might see in the northern continents a true 

 cause of increased barometric pressure at the sea-level 

 of the northern hemisphere. 



We have, however, not only no evidence that such 

 an arrangement exists, but very strong evidence of an 

 atmospheric circulation which carries air from hemi- 

 sphere to hemisphere, and mixes in the most intimate 



