PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA - 23 
casting models, whereas other areas are more or less white spots on 
the map of successful model building. This point of view goes to 
the core of our proceedings: The very theme of our Study Week 
makes a challenge to econometrics to give evidence how far our 
science has reached towards the goal of valid explanatory and fore- 
casting models in two areas of paramount importance: economic 
growth and business cycles. 
I should like to develop this point a bit more. Let us take very 
briefly one example: meteorology. Fact-finding includes of course 
everything about the weather. The theory of meteorology has deve- 
loped gradually, but it is rather recently that explanatory models 
were built that could be exploited for reliable forecasts. The breakth- 
rough was the thermodynamic theory of cyclones, founded by the 
Bergen school around 1918, with J. BJERKNES and T. BERGERON as 
leading names. 
Cold r 
——, 
Warm 
With reference to the graph, the theory says that cyclo- 
nes tend to develop on the border between cold polar air and 
warm equatorial air; on the northern hemisphere the cyclones move 
eastwards, go toward culmination in some few days, and then dissolve 
in an occlusion of the warm front and the cold front. On the basis 
of the thermodynamic theory the forecasting techniques have gra- 
dually improved and become more reliable. Here is a clearcut case 
where we have seen an important development in model building in 
the short span of some twenty years, during which all three levels 
of model building have come into the picture. Two points of general 
‘1] Stone - pag. 092