40 SECTIONAL ADDRESSES 
forward in 1929 a theory of a four-year period, based on the cooling 
effect of the widespread growth of luxuriant vegetation produced by the 
rainfall in areas that were parched. I believe that the theory has not 
been adopted officially. 
When we turn from the tropical and subtropical to the temperate 
regions, where the persistence of conditions is in general conspicuously 
1922/3 23/4 24/5 25/6 2b/7 27/8 28/9 2/30 30f1 31 32/3 
poser 
ACTUAL 
S. RHODESIA RAINFALL, OCT~APR. 
Fic. 12. 
smaller, we must expect greater difficulties in making long-range forecasts. 
In America the relations of weather and crops have probably been worked 
out more scientifically than in any other country, so that the commercial 
value of reliable predicting has long been recognised ; and not only by 
farmers, but by those interested in water supply, in power schemes, 
in transport and in commerce generally. Thus one of the Californian 
hydro-electric companies makes its own forecasts, because it may spend 
four million dollars more for crude oil in a dry than in a wet year. Ina 
country of exuberant vitality it is not surprising that many efforts should 
