﻿346 Mr. J. K. Laughton on Barometric Differences 



Although very much larger, and having boundaries of a very 

 different nature, the Antarctic is as confined as the valleys of 

 Switzerland ; and the friction which is continually going on at 

 its exterior limits between the strong westerly winds and the still 

 air inside must produce, or tend to produce, the same effect. It 

 is impossible to form any reliable estimate of the effect which is 

 really produced, because we know neither the absolute depres- 

 sion of the barometer, nor how much of it is due to the centri- 

 fugal tendency established by the rotation of the earth ; but 

 since the Swiss observations in the localities I have referred to 

 give 5 to 6 millimetres as the barometric depression which pre- 

 cedes and accompanies the setting in of the Fohn, we may con- 

 sider that two-tenths of an inch is the most that can be attributed 

 to a similar cause in the Antarctic. 



Into the comparative vacuum thus formed we might expect a 

 certain portion of the air from outside to flow ; and some does 

 seem to do so, and to form a series of curling eddies on the inner 

 or southern margin of the west winds; but the influence which 

 these exercise on the west winds themselves is exceedingly small 

 and altogether imperceptible. As, then, the west winds blow 

 incessantly, and incessantly abstract air from the Antarctic, and 

 yet the pressure of air in the Antarctic remains nearly constant, 

 it is clear that air must be continually, or at least frequently 

 poured in to take the place of that which is carried away. We 

 have no evidence as to where or how this supply is given ; that 

 offered by the northerly winds does not appear sufficient ; and if 

 on such a point, in the utter absence of any positive knowledge, 

 conjecture may be permitted, I would say that it seems not im- 

 possible that, in consequence of the continual removal of air from 

 the Antarctic, the atmospheric strata outside, at some unknown 

 distance from the surface of the earth, are left so much above the 

 strata of equal density inside that the eastward force is overcome 

 by the pressure southwards, and that they flow over. A supply 

 so given at the top of the column would clearly be regulated by 

 the quantity of air previously withdrawn from the bottom ; and 

 when the great current of westerly wind has been blowing stronger 

 and tearing away more air than usual, more air than usual will 

 be poured in above; so that, after the strength of the westerly 

 gale has abated, there will be an excess of air within the confined 

 circle, which being compelled by the centrifugal tendency to move 

 outwards, may show itself in some locality as a southerly gale. 



The arctic regions differ from the antarctic in many ways, and 

 essentially in this ; that whilst the Antarctic is open, bounded 

 only by the westerly winds sweeping over an unbroken sea, the 

 Arctic is shut in by the continental shores which extend round 

 almost its entire circumference. The westerly winds, again, of 



