SOME WORLD'S WEATHER PROBLEMS 213 



in the course of three or four years. This alternate trans- 

 portation of air from the east to the west, and vice versa, must 

 affect to some degree, also alternately, the existing currents 

 of air. In tropical regions, such as India, for instance, the south- 

 west monsoon — the rain-bearing current which mainly fertilises 

 that vast country — must suffer some change, and it only requires 

 a slight deflection from its mean course to prevent this land from 

 receiving during its summer months its main water supply 

 requirements. 



Even countries which are not in the zone of steady trade or 

 monsoon winds, but whose weather is the resultant of the 

 passage of cyclones and anticyclones — eddies in the general 

 atmospheric circulation — must also be affected. 



The middle and southern portion of Australia, for instance, 

 is in the path of great air eddies of high pressure — or " anti- 

 cycles " as they are called — travelling from west to east, while 

 New Zealand, farther to the south, is in the track of a series 

 of cyclones — areas of low pressure — which are trying to wedge 

 themselves, sometimes with success, between the Australian 

 anticyclones. 



Slight changes in the paths or intensity of these air systems 

 which must and do occur in consequence of the alternate excess 

 and deficiency of pressure caused by this barometric see-saw, alter 

 very considerably the distribution of rainfall over this large tract 

 of land in that part of the world. 



Similar changes occur in the British Isles. The paths of low 

 pressure areas are sometimes to the north of Scotland, while at 

 other times they are, on the average, situated well across the 

 middle of England. 



In the former case they are pushed to these higher latitudes by 

 the high pressure areas traversing the Continent, since cyclones 

 always move on the fringe of such high pressure systems. In 

 the latter case, the course of the high pressure systems being 

 situated farther to the south, the cyclones follow suit. 



This movement of the tracks is also bound up with the 

 world-wide barometric see-saw ; Great Britain being, however, 

 so far removed from the main central regions of action — namely, 

 those enclosed by the smaller oval curves shown on the map — 

 presents consecutive changes of three years' rather than four 

 years' duration. 



While it is easy to represent with considerable accuracy the 



