238 THE ANTARCTIC. 



6o° S. noted only one single day when seven-eighths of the 

 sky was cloudless ; at other times dense masses of clouds 

 commonly filled the upper strata of the air, whilst impene- 

 trable mist and fog lay over the sea, and fully shrouded 

 the ice masses to the great danger of navigation. 

 Reasoning from analogy of the Arctic regions, one is led 

 to assume that the cloud covering is largely reduced in 

 the winter months, so that the heavy clouds of summer 

 obstruct the admission of the sun's rays, and the clearer 

 sky in winter favours excessive radiation, both circum- 

 stances thus combining to reduce the temperature. 



The distribution of Atmospheric Pressure presents 

 phenomena as extreme as that of temperature. As- 

 suming the existence of a water surface right to the 

 Pole, atmospheric pressure ought, according to Hann's 

 theory, to diminish as the latitude increases. As a matter 

 of fact the Antarctic regions seem to exhibit permanent 

 depressions of the barometer, such as occur elsewhere 

 only in the centre of progressive cyclones. On the basis 

 of the observations made by Ross, Hann has calculated 

 the mean barometric values of the southern summer in 

 Ross Sea for the years 1840-41 and 1841-42 as follows : — 



According to the observations of Wilkes (who reduced 

 his values to latitude 45° 1 ), the mean values for January 

 and February, 1840, for the coast of Wilkes Land (i.e., 

 for about the Antarctic circle) amount to 28*846 inches, 

 consequently to considerably less than even the values 

 recorded by Ross. This is probably due to the fact 

 that alone the northern coast of W 7 ilkes Land a main 

 track of cyclones seems to lie. Observations made east 

 of Graham's Land show a somewhat higher pressure. 



1 We are not informed if Wilkes' barometric measures have been 

 •corrected according to temperature. 



