The Low Barometer of the Antarctic Temperate Zone. 339 



that an elastic fluid should be influenced in the manner sug- 

 gested. In fact, if this happened we should no longer be 

 enabled to determine the heights of mountains by barometric 

 observations; for according to the theory the air should 

 extend to a gTeater height above mountains than above plains; 

 and as regards comparison between a barometer at the foot of 

 a mountain and one at the summit, we might argue that the 

 barometer in the valley, compared with a barometer at the 

 same level in a plane district, " is pressed upon, not only by 

 the air clear of the mountain tops, but also by the air confined 

 within the valley," so that the altitude over the valley is 

 greater by some hundreds of feet than the altitude over a 

 plain at the same level as the valley ; and thus, before we 

 could determine the height of the mountain above the level of 

 the plain, we should have to determine the exact effects due to 

 the confinement of the air in the valley. We know that, on 

 the contrary, the average barometric pressure in the most 

 confined valley does not differ appreciably from the average 

 pressure over the most widely extended plain at the same 

 level. 



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 con- 

 taining 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. Nov/ if we suppose a globe covered all 

 over by water to be surrounded by a perfectly uniform atmo- 

 spheric 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 conti- 

 nents 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 hemisphere to hemisphere, 

 and mixes in the most intimate manner the whole mass of 



