POPULATION OF THE UNITED STATES 



383 



close approximation of the value of this constant may be obtained. To 

 take another illustration: The relation between the velocity of water 

 flow to its depth below the surface in a river is not a constant one, but 



i8oo laio 



■820 1830 



iSfiO 



v e a b 



i860 1890 



Fig. 1. 



repeated measurements have shown that by the use of an empirical 

 formula containing certain constants, this relation may be quite well 

 established. On the other hand, the relation between the space passed 

 over by a freely falling body and the time of its fall is a perfectly 



