Page:Popular Science Monthly Volume 76.djvu/387

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383
POPULATION OF THE UNITED STATES

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

 
PSM V76 D387 Population increase in the us and europe 1800 to 1900.png
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