16 



The interchange of air between equatorial and polar regions may be effected through the intermediary 

 of nnticyclonic circulations, albeit these high-pressure systems are permanent ; and, in the opinion of the 

 writer, the temperate zones are bridged in this manner. The heat thus transmitted from low to high 

 latitudes, while mitigating in some measure the severity of the climate of northern Antarctica, may also 

 contribute towards the disruption of ice from outlying ice barriers. Similarly the introduction of cool air 

 from high to low latitudes doubtless exercises an ameliorating effect upon tropical regions through the 

 agency of the trade winds. 



THE RATE OF TRAVEL OF CYCLONIC DEPRESSIONS. 



To revert to the cyclonic depressions experienced in far southern seas ; it has been ascertained* that in 

 the Southern Ocean, between the meridians of 10 and 140 E., the strongest gales experienced near the 

 centre of a cyclonic depression blow in its front, from between North-east and North-west, and that the 

 wind moderates when it backs to Westward, as pressure increases ; whereas gales that occur within the 

 northern segment of a cyclonic depression remote from its centre attain their highest velocity in rear of 

 the line of lowest barometer readings, from some point to the South of West, usually from about South- 

 west, while pressure is rapidly recovering, the wind having been comparatively moderate from North- 

 westward while pressure was diminishing. 



The high velocity of the wind on the outskirts of a depression in rear of its centre may be attributed 

 partly to the proximity of the high-pressure belt situated to the north of the Westerly air current, and 

 partly to the rapid recovery of pressure that takes place by replenishment of air from a polar quarter. 



The paths followed by centres of cyclonic depressions vary with the season ; the parallels along which 

 they move to the eastward depending mainly upon the position of the tropical high-pressure belt which 

 consists, for the most part, of the southern anticyclones of the great oceans ; which, in addition to their 

 annual east and west expansion and contraction, have a motion North and South. The amplitude of 

 these seasonal oscillations appears to vary in different years ; and it has been suggested that the tropical 

 anticyclones may have a cycle of long period as well as a seasonal movement. This point will be referred 

 to later. 



The average paths of the centres of cyclonic depressions depicted on the daily charts under notice have 

 been estimated and grouped. The method employed in their construction was as follows : The probable 

 positions of the centres of all depressions, that can be identified day after day on four or more consecutive 

 daily charts, were plotted on skeleton charts. Four charts were used for this purpose, one for each 

 season of the year ; and the several positions allotted to the centre of each identical system was joined by 

 a line. Parallel to, and bisecting as nearly as possible the areas covered by the paths indicated, a line was 

 drawn to represent the average path. 



In grouping the paths in seasons, the months of September, October, November are selected to represent 

 Spring ; December, January, February : Summer ; March, April, May : Autumn ; and June, July, August : 

 Winter. 



The results obtained are as follow : The average path of all central areas of depression charted for the 

 entire period, October, 1901, to March, 1904, is found to have been in about the 52nd parallel. Between 

 the meridians of 20 E. and 150 E., that is to say, over the South Indian division of the Southern Ocean, it 

 was between the 49th and 50th parallels; and between 150 E. and 70 W., the South Pacific division, in 

 about the 55th. 



The South Atlantic division of the ocean, embraced by the meridians 70 W. and 20 E., is rarely visited 

 by ships, except on the western side ; the observations relating to that portion of the Southern Ocean are, 

 therefore, limited almost exclusively to that side, and to that side only the average paths estimated for 

 this division of the ocean refer. 



During each of the seasons the centres of depression after passing the meridian of Joinville Island, about 

 56 W., either struck eastward or south-eastward ; or moved east-north-eastward or north-eastward. 



In the Spring of the year the average path taken by centres of depression is found to have been between 



* ' The Trucks of Occau Wind Systems in Transit across Australasia,' quoted on p. 9. 



