212 SCIENCE PROGRESS 



naturally ask whether the existence of this barometric see-saw 

 is going to help meteorology? The reply is that it will help 

 this science very materially. 



In the first instance meteorologists have, for the most part, 

 been led to consider atmospheric changes as occurring mainly 

 between the equator and the poles ; but here we have a distinct 

 east and west action taking place. This must in the future 

 be legislated for. 



Again, as we are in the presence of air being transferred 

 from one half of the world to the other half, along a parallel of 

 latitude, the places in each of these halves must be closely allied 

 meteorologically. In fact we have here possibly an important 

 clue to the close connection between the meteorological be- 

 haviour of regions which are widely separated. For instance, it 

 is only quite recently that Sir John Eliot pointed out that the 

 drought in the Indian region during the years 1895 to 1902 

 was a more or less general meteorological feature of the whole 

 area, including Abyssinia, East and South Africa, Persia, 

 Baluchistan, Afghanistan, and probably Tibet, and the greater 

 part or the whole of Australia. A glance at the map here given 

 shows that all these localities fall in the eastern hemisphere 

 portion of the see-saw, so that they should be affected similarly. 



Now a very important point in relation to this transference 

 of air from one hemisphere to the other is the interval of time 

 occupied by each of these barometric surges. One to-and-fro 

 motion occupies nearly, but not quite, four years on the average ; 

 but the intervals are not all equal, so that this value is only 

 approximately true. 



It must be understood, however, that this particular four-year 

 variation applies only to the more central parts of the two 

 reciprocating areas, such as, for instance, those enclosed in 

 the smaller dotted and continuous lines shown in the map 

 to which reference has previously been made. Outside of 

 these the variation seems to be shortened as regards duration, 

 and in such cases as the British Isles, Europe, Canada, United 

 States, etc., the changes exhibit more of a three than a four-year 

 variation. In these instances also the intervals are not perfectly 

 regular. 



There is, I think now, little doubt that this pressure oscilla- 

 tion dominates, and is therefore responsible for, the very 

 different types of weather that are experienced at any one place 



