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II. THE rojj: of modern hydraulic models 



Hydraulic models as tbey are understood and enployed today 

 are the product of several decades of experimental development. 

 There are no comprehensive mathematical relationships to express 

 the entire range of correspondence between models and their pro- 

 totypes . Lacking these, the design and operation of hydraulic 

 models is an art and the results obtained are necessarily subject 

 to interpretation. The process of interpretation is frequently 

 qiialitative or categorical in the "is" or "is not" sense since a 

 full accounting of all the physical properties of a fluid system 

 is not always possible. Within these limitations, however, the 

 use of models has great interpretive value . Today, most important 

 problems in fluid engineering are solved through canbined consid- 

 eration of model experiments and mathematical analysis of the 

 known physical factors. 



The function of a model is essentially that of an analogue 

 computer. Vl/hen a geometrically extensive or lengthy series of 

 hydraulic events involves a great number of complex factors it is 

 too difficult for the human mind and too laborious for exact 

 mathematical analysis to cope with them all simultaneously. Usu- 

 ally, more intelligible results can be obtained if a similar sys- 

 tem is constructed and studied under controlled laboratory 

 conditions. 



The degree of similarity between a model and its prototype 



