478 



and particularly was this so in the journey made by Lieutenant EOYDS to the south-east across the open 

 Ice Barrier. Throughout this journey the aneroid was read at frequent and regular intervals, and the 

 observations have been dealt with in the following manner. The journey out and home was broken up 

 into distance-intervals of 20 miles each, and all the barometric observations made whilst each distance- 

 interval was being traversed were moaned and then compared with the mean of the corresponding readings 

 of the barometer made at the ship. In this way it was sought to smooth out the casual fluctuations of 

 pressure and to get an approximately correct mean barometric difference between the ship and the mean 

 position of the travellers for the time occupied in traversing each 20-mile section of the journey. 



The differences obtained from a comparison of the two sets of means when plotted in a curve* exhibit a 

 very uniform increase outwards to the furthest point reached, and a similarly uniform diminution on the 

 return journey, the reading of the barometer at the ship being the higher one throughout. It is, however, 

 tolerably certain, from various considerations, that the general level of the Barrier rises to the southward, 

 and this in itself would of cours.6 account for a decrease of pressure in that direction and a consequent 

 increase in the difference between the two means as the travellers increased their distance from the ship. 

 The question then arises whether this steady increase of difference with distance is not wholly due to 

 increase of height above sea level, and whether if it had been possible to reduce the readings made on the 

 ice to sea level there would not have been shown an increase of pressure to the southward instead of a 

 decrease ? 



If we may assume an average rise of surface level of the Barrier ice of about 2 feet per mile along the 

 line followed by the travellers (which is probably well within the actual amount), and if we reduce the 

 barometer readings in accordance with that assumption, such a sea-level increase of pressure is then found 

 to exist, and not only does the decrease of a little less than three-tenths of an inch in the 170 miles then 

 disappear, but an increase of over a tenth of an inch takes its place. 



It has been elsewhere pointed out (p. 492) that this conclusion receives considerable support from the fact 

 that it supplies the barometric gradient required to account for the persistent easterly wind which is so 

 marked a feature of the air circulation of these high latitudes. 



The track pursued in two other journeys similarly dealt with followed somewhat closely the line of high 

 mountains which stretch away to the southward along the western boundary of the Ice Barrier, and it is 

 probable that on those journeys greater and less regular differences of level were met with. At any rate, 

 the observations made during those journeys do not yield results quite so uniform as that obtained from 

 the observations on the journey across the open Barrier, quite away from the land, although in their main 

 features they are very similar ; and this may probably be due partly to the travellers having experienced 

 on the southern journeys more abrupt change of level, and also to some extent to the fact that the readings 

 of the aneroid were not made either so frequently or so regularly as on Lieutenant EOYDS' journey. It 

 may, perhaps, be of interest to notice that at the 130-mile distance-interval of Lieutenant EOYDS' journey 

 there is, both on the outward and the homeward journey, a decrease of the barometric difference amounting 

 to 0-03 inch or 0-04 inch, indicating a slight decrease of about 30 feet in the height of the Barrier floor at 

 that distance. The repetition of the irregularity in the curve on the return journey not only confirms the 

 existence of a depression of ice level at that spot, but also seems to support the method adopted for dealing 

 with the observations. 



HANN, in his 'Handbook of Climatology' (English edition, translated by E. DE C. WARD, p. 217), refers 

 to the distribution of mean pressure at different latitudes, and quotes FERREL'S value for each 10 down to 

 latitude 70 S. These values indicate a progressive decrease of pressure with increase of latitude south of 

 30' S., the mean pressure at 70, which is the highest latitude quoted in the table, being given as 

 29 056 inches. 



Map No. 51, in the ' Report of the " Challenger " Expedition/ showing the isobaric lines of the globe for 

 the year, exhibits a similar decrease of pressure down to latitude 60 S., which is the most southern position 

 for which an isobar is given, and the value of the pressure for that parallel is 29 -30 inches. 



Compared with the values given in Table I., that quoted by HANN for latitude 70 is too low, and it is 

 clear that the decrease of pressure does not continue southward so far. Dr. SHAW, from a discussion of 



* See 6g. 1, p. 491, " Discussion of Wind Observations." 



