PRESIDENTIAL ADDRESS. LXXIII 



Presumably, increase in centrifugal force may partly explain the 

 lower barometric pressure in Antarctica in winter as compared with 

 summer, but are there any other contributing factors ? Certainly 

 the flotation effect of aqueous vapor cannot assist, inasmuch as 

 obviously in winter, when the air is colder, there would be less 

 aqueous vapor along given latitudes than in summer, when the air 

 is warmer. The vapor factor would, therefore, tend to increase 

 pressure in winter, rather than to diminish it. I would venture to 

 suggest very tentatively that another factor contributing to a 

 lessening of atmospheric pressure in winter may be of the nature 

 of an actual thinning of the atmosphere. 



The question appears to be one of hydro-dynamics, and may 

 be rather intricate. The suggestion may be made that the thinning 

 is brought about by a lag in the movement of the upper atmo- 

 sphere as compared with that of the lower. For example, in the 

 case of the blizzards, it would appear that the masses of cold air 

 over the Antarctic slip away from underneath the air of the 

 circum-polar whirl at such a rate that the inflowing air from the 

 north is unable to overtake the demand for air near the Pole to 

 take the place of what has been lost in the blizzard. This is 

 perhaps the explanation of the phenomenon of the sinking of the 

 upper air currents from an altitude of 15,000 feet to that of about 

 12,000 feet at Erebus during the progress of a severe blizzard. 



As regards (b), the answer usually given to explain the 

 diminished atmospheric pressure at the Poles as compared with 

 that at the Equator is that it is due to the increase of centrifugal 

 force of air masses seeking to conserve their moment of momentum 

 as they move polewards. This explanation must be provisionally 

 accepted, but it is not necessarily the only explanation. The lower 

 atmospheric pressure near 60° S. as compared with 60° N. is 

 chiefly due probably to the greater preponderance of sea over land 

 in the Southern Hemisphere near that latitude as compared with 

 the northern, and the consequent less frictional resistance to wind 

 currents, in 60° S., as compared with 60° N. 



Possibly here, too, there is a hydraulic factor tending to locally 

 thin the atmosphere during the normal out-rush of the anticyclonic 

 wind as it spirals first in a N. by W., and eventually in a W. by 

 N. direction. That gigantic refrigerator, the Antarctic Continent, 

 may, during blizzard time or in the itnervals between the blizzards, 

 throw air off its shoulders at such a rapid rate that the incoming 

 air is unable to overtake the demand, and so seldom, if ever, comes 

 into gravitative adjustment. This assumed thinning of the atmo- 

 sphere by lessening the thickness of the earth's air blanket in the 

 direction of the Antarctic Circle and the Pole would tend to 

 increase the cold in those regions by allowing heat to escape more 

 rapidly where the earth's aerial clothing is thin than where it is 

 thick. This suggestion is, of course, purely speculative, but the 



