value fairly quickly. On the other hand, you can go to the construction of a model of 

 the river, or, what is the equivalent of a model, use of a digital computer. You are 

 obviously best-off to use the model, if you want an accurate answer. 



/. J. Stoker 



I would say that if you once decide to solve these problems by integrating differ- 

 ential equations by one scheme or another, then the biggest job is the processing of 

 the data, obtaining the resistance coefficient, getting the cross-section areas, coding all 

 of this, and setting up a procedure. It could be that by using simplified differential 

 equations, the whole thing might be done on a smaller computing machine, which would 

 be less costly in its operation. On the other hand, as I have pointed out, it doesn't take 

 very long anyway to make these calculations. 



M. J. Lighthill 



Three years! 



44 



