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XI. NOTES ON THE OBSERVATIONS OF BAROMETRIC PRESSURE. 



BY 



R. H. CURTIS. 



I. THE YEARLY MEAN PRESSURE. 



NOTWITHSTANDING the great interest which attaches to the distribution of barometric pressure over the 

 South Polar regions, our knowledge of the subject has hitherto been to a great extent hypothetical owing 

 to the fewness of the observations we possessed. It is only quite recently that observations covering an 

 entire year have become available. All those previously received were made by navigators (in most cases 

 exploring the region) who passed quickly from point to point, so that their observations are a good deal 

 scattered both as regards locality and time, although in every case they refer exclusively to the summer 

 months of the year, in which alone the Antarctic seas are open to navigation. 



It was not until the year 1898-9 that a series of observations embracing a complete year was made 

 within the Antarctic circle, and the whole of the information we now possess, relative to the sequence of 

 climate in the Antarctic throughout the year, from that time up to the return of the " Discovery," is set 

 out in Table I. As regards the stations at which these four series of observations were made, it should be 



TABLE I. -Mean Height of the Barometer reduced to Sea Level, to 32 F., and to Latitude 45. 



noted that whilst two of them Cape Adare and the "Discovery's" Winter Quarters are not very greatly 

 separated as regards latitude, and are almost on the same meridian of longitude, the other two are remote 

 both from Ross Island and from each other, their distance in longitude being roughly 90 from Ross Island 

 and 180 from each other. 



The barometers used on board the " Discovery " were verified at the Kew Observatory of the National 

 Physical Laboratory both before the Expedition sailed and subsequently to its return. During the greater 

 part of the sojourn at Winter Quarters they were read every second hour from midnight to midnight, but 

 towards the close of the second year the number of eye observations became reduced and the omissions 

 have been supplied from the records of one of the self-recording aneroids, after a suitable correction had 

 been applied to bring them into accordance with the readings of the mercurial barometer when corrected 

 and reduced to sea level and to temperature 32 F. 



Each of the four series of observations given in the table began in February or the beginning of March, 

 and the mean heights of the barometer for the ensuing twelve months at the places represented were as 

 follows, the readings being in each case reduced to latitude 45, as well as for temperature and to sea level. 



We do not know how nearly the mean of one year's observations approximates to the true annual mean 

 pressure for these regions, and it will be noticed that the individual means for the two years of the 

 "Discovery's" stay differ by as much as 0-125 inch. A comparison of the mean annual pressure at 



