GRAVITY PROMOTES UNDERGROUND CIRCULATION. 147 



But in earth movements and water circulation alike, all the elements of the 

 movement must be taken into account. The downward movement of a 

 greater mass of earth or water may result in the upward movement of a 

 lesser mass. The upward movements of water dependent upon downward 

 movements of other water are of relatively greater importance in the water 

 circulation than are the upward movements of rocks consequent upon 

 downward movements of larger masses of material in earth movements. 



Indeed, it will be seen that commonly the circulation of a system of 

 ground water in the belt of cementation involves both downward-moving 

 and upward-moving masses. In such systems of ground-water circulation 

 gravity is effective in the movement in proportion to the head. Head is 

 due to the fact that the water entering the ground at a certain level, after a 

 short or long underground journey, issues at a lower level. 



Also where there is a difference in the density of the two columns due 

 to difference in the amount of material held in solution, gravity promotes 

 circulation independently of head, the column holding more salts being 

 pulled down and the lighter column driven upward. Probably the amount 

 of material in solution is usually not so great as to make this an important 

 factor in the process, but in salt regions it may be important. The density of 

 the water of the sea as compared with fresh water is 1.02765 to 1.02795," and 

 the density of a saturated solution of sodium chloride at 4° C, as experi- 

 mentally determined by Mr. S. H. Ball, is 1.2063. Of course in actual cases 

 such differences as these are not found, for both columns are sure to have 

 salts in solution ; but where springs empty under the sea the first case is 

 approached. In such instances, the increased density of the sea water 

 opposes the head of the lighter stream of relatively pure water. 



Heat. — Change in temperature may result in the expansion and contrac- 

 tion of water, and such changes in volume necessarily involve some move- 

 ment. The volume of water varies as the temperature. Taking the 

 volume of water at 4° C. as 1, its volume at 50° C. is 1.0120, at 75° C. is 

 1.0258, and at 100° C. is 1.0432." Therefore the increase in the 

 temperature of underground water may increase its volume and lessen its 

 density as much as 4 per cent without exceeding its boiling point at atmos- 

 pheric pressure, and a difference in the density of two columns by 1 per 



a Bischof, Gustav, Elements of chemical and physical geology, translated by Paul and Drummond, 

 Harrison & Sons, London, vol. 1, 1854, p. 97. 



6 Austin, L. W., and Thwing, C. B., Exercises in physical measurements, Allyn & Bacon, Boston, 

 1895, p. 151. 



