477 



Valencia, in the south-west of Ireland, for each of '20 consecutive years with the mean for 35 years gave an 

 average difference of + 0-03 inch, and an extreme difference of 0-112 inch, whilst a similar comparison of 

 mean values for Kew for the same period gave an average difference of 0-025 inch and an extreme 

 difference of only 0-067 inch. Possibly in the Antarctic the variation of the mean pressure of any one 

 year from the true mean may be liable to a greater error than either of these we have no sure information 

 upon the point, but probably we shall not greatly err if we assume for the values we are now dealing with 

 a possible error of 060 inch. 



The annual means for the four places named in the table confirm the inference which seems inevitable 

 from the marked prevalence of easterly winds at three of them, that barometric pressure must be relatively 

 high over the regions approximating to the Southern Pole. 



Taking first for comparison the " Gauss " yearly mean for latitude 66 and the " Discovery's" mean for 

 the same twelve months for latitude 78, we find the mean pressure at the more southern station to be 

 0-219 inch higher than at the northern position, giving a gradient, if we may disregard longitude and 

 consider the difference in latitude alone, of 0-018 inch for each degree of latitude. The Cape Adare mean 

 for latitude 71, dealt with in a similar way, but comparing it with the "Discovery" mean for the two 

 years, gives a difference of 0-022 inch per degree of latitude, which is practically the same as that given 

 by comparing the " Discovery " and " Gauss " values. 



That the "Belgica's" average pressure for very nearly the same (mean) latitude as Cape Adare (71) 

 gives a somewhat different result is not very surprising when we remember that it is not only for a 

 different twelve months from that of the " Discovery," but also for the somewhat extensive area covered 

 by her drift during the time. Her mean pressure for the year is 0-173 inch higher than the Cape Adare 

 mean, and only 0-046 inch lower than the mean for the "Discovery's" first year, which was considerably 

 (0 - 125 inch) higher than that for her second year. This difference yields a gradient per degree of latitude 

 less than half that obtained by comparing the means of the other stations. 



The " Belgica's " high mean pressure for the year is, however, chiefly due to the relatively high means 

 for the four winter months, which is a feature not exhibited at either of the other stations ; indeed, at the 

 " Gauss " the mean pressure for the winter was considerably lower than that for the summer. Assuming 

 the seasonal march of pressure to have been approximately normal at each of the stations, it is conceivable 

 that the higher winter pressure at the " Belgica " may have been related to a pole of cold caused by 

 radiation from large land masses somewhere in her neighbourhood, leading to some dislocation of the 

 isobars, and incidentally to the greater diversity of wind direction which she experienced. It has been 

 pointed out that she was far to the eastward of the " Discovery," and as very little is known of the 

 physical features of the intervening region, there is plenty of room for conjecture as to the distribution of 

 land and sea areas within it. 



Before leaving this part of the subject it may be well to refer to the results of the barometer obser- 

 vations made on the various sledge journeys which were undertaken by the Expedition during its stay at 

 Ross Island. 



These journeys varied both in direction and in duration, and barometric observations were made on 

 nearly all of them. The instrument generally used was an aneroid, of which the Expedition possessed 

 several, but on most of the journeys the readings of the aneroid were supplemented by hypsometric 

 observations. Prior to leaving England the aneroids had been verified at the Kew Observatory, but when 

 they were brought into use in the Antarctic it was found that each instrument had developed errors 

 very different from those determined for it at Kew. This fact introduces an element of uncertainty into 

 the observations which is not entirely eliminated by the comparisons which can be made of the aneroid 

 readings with the readings of the ship's mercurial barometers at the starting and on the return of the 

 party. But the greatest obstacle in the way of ascertaining the pressure gradient from the barometric 

 observations made on the several journeys arises from the difficulty of deciding how far the differences 

 observed between the observations made at the ship and those made simultaneously at a distance were due 

 to change in height above sea level, and how far to purely meteorological causes ; and in some instances, 

 where the changes of level are known to have been very large from day to day, the difficulty becomes 

 insuperable. In two or three journeys, however, the conditions were more favourable than in the others, 



