to Ground Water along Coasts. 2S3 



Law of equilibrium between fresh and salt water. 



The reason that fresh water can exist in close proximity 

 to sea water on small islands composed of freely porous 

 material is that it has a lower specific gravity and so rests 

 upon the underlying salt water. The slow rate at which 

 water percolates through the crevices in rocks and the 

 equally slow rate of diffusion of salt through rocks pre- 

 vent the rapid mingling of the fresh water with the salt 

 water, and the continued increment from rains replaces 

 that which is lost. The same conditions exist on main- 

 land coasts. As a result, the fresh and salt water reach 

 static equilibrium and assume positions which are depend- 

 ent on relative specific gravities, the higher column of 

 fresh water balancing the heavier sea water. This prin- 

 ciple was first recognized by Badon Ghyben, 8 a Dutch 

 captain of engineers, as the result of work clone on the 

 coast of Holland; it was later enunciated by Herzberg, 

 apparently without knowledge of the work of Badon 

 Ghyben. Herzberg 9 found in drilling wells on Xorcl- 

 erney, one of the East Frisian Islands ofT the German 

 coast, that the depth to salt water was roughly a function 

 of the height of the water table above mean sea level and 

 of the density of the water of the Xorth Sea. Figure 3 

 shows the application of his theory. 



Let H be the total thickness of the fresh water. 

 Let h be the depth of fresh water below mean sea-level. 

 Let t be the height of fresh water above mean sea-level. 

 Then H = h + t. 



But the column of fresh water H must be balanced by 

 a column of salt water h in order to maintain equilibrium. 

 "Wherefore, if g is the specific gravity of sea water (and 

 if the specific gravity of fresh ground water is assumed 

 to be 1), 



H =z h -{- t = hg, whence 



» = ' 7 



9-t 



6 Badon Ghyben, W. : Xota in verband met de voorgenomen put boring 

 nabij Amsterdam, Tijdschrift van het Konirklijk Instituut van Ingenieurs, 

 Instituuts jaar 1888-1889, p. 21, 's-Gravenhage. Referred to by Eugene 

 Dubois. 



9 Herzberg, Baurat: Die Wasserversorgung einiger Xordseebader, Jour. 

 Gasbeleuchtung und Wasserversorgung, Jahrg. 14, Miinchen, 1901. 



