SECT. 2] LABGE-SCALE INTERACTIONS 287 



seek critical parts of it for careful study. Thus they must try to pose questions 

 which isolate tractable features of the complexity which can be treated under 

 relatively controlled conditions. This usually means recourse to idealized, 

 simplified situations : situations which may be realized only in mathematical 

 or laboratory models, but which strive to select and relate the important 

 physical processes which govern some phases and motion scales of the natural 

 system. Rarely, but often enough to be of inestimable value, nature herself 

 performs an experiment under partially controlled conditions, as we have seen 

 in some of the trade-wind cases ; one of the main skills of the earth scientist lies 

 in being able to recognize and exploit these, using them to guide his measure- 

 ment programs and as a framework within which to relate the results. 



As in all geophysical studies, we are faced with constant compromise and 

 decision between the practical and the apparently impracticable, the extensive 

 and the intensive effort. If the air-sea interaction picture developed here is any 

 fair representation, then we find that, in the long run, the intensive study has 

 the most extensive value to our understanding, and that the apparently im- 

 practicable investigations of controlled or idealized situations are the in- 

 dispensable foundations upon which practical developments depend. 



To evolve a basic mechanics of turbulent fluid motions, the sea and air 

 around us provide stimulation, a proving laboratory, and a necessary practical 

 motivation in terms of the benefit to humanity. But our progress in the study 

 of natural planetary fluids can develop little farther without concurrent 

 building of these foundations, to which a large fraction of our effort must be 

 devoted. 



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