2i 4 SCIENCE PROGRESS 



changes of air pressure from year to year over a large area, by 

 means of a curve constructed from the readings of one baro- 

 meter, it is not generally possible to determine accurately the 

 rainfall changes from the records of one rain-gauge. The 

 amount of rain collected in a rain-gauge is so dependent on its 

 position and the local configuration of the land, that, in order to 

 form some idea of the variation from year to year over a large 

 area, the data gathered from numerous well-scattered instru- 

 ments have to be combined. Granting, therefore, that baro- 

 metric curves represent the pressure facts over large stretches 

 of country, much more accurately than rainfall curves illustrate 

 the actual variations of rainfall, the reader's attention may be 

 drawn to the set of pressure and rainfall curves brought together 

 in the accompanying figure (fig. 2). 



Now the rainfall curves here shown represent the variations 

 from year to year at places where a reduction of pressure 

 corresponds to an increase of rainfall. In other words, for 

 those comparatively small areas over which the fall of rain is 

 here taken into account, the greater the pressure the less the 

 rainfall, and the lower the pressure the greater the rainfall. 

 The function of an excess of high pressure, then, in those 

 areas, is to obstruct as much as possible the passage of the 

 rain-bearing air currents, which results either in the deflection 

 of the moist winds from these regions or in a great reduction 

 in their normal strength. If the pressure be abnormally low, 

 we are presented with the most favourable conditions for great 

 rainfall. In the curves here shown, then, those illustrating the 

 pressure changes are inverted, so that their highest and lowest 

 points will synchronise with those representing the times of 

 greatest and least rainfall. The peaks in the pressure curves 

 indicate, therefore, the years when the pressure was very low ; 

 and it will be seen that they correspond remarkably closely 

 with the highest points of the rainfall curves, which pick out the 

 wettest years for the regions investigated. 



To illustrate the generality of this behaviour between 

 pressure and rainfall for widely separated areas, typical instances 

 are given for India, Australia, and England. Further, in order 

 not to present rainfall curves which may only show fortuitous 

 agreements with the inverted pressure curves, two absolutely 

 independent sets of rainfall curves are shown with each pressure 

 curve. 



