THE ARTESIAN WATER SUPPLY OF EASTERN FLORIDA. 103 
(1) . Porosity of the material. 
(2) Size of the pores in the water-bearing medium. 
(3) Pressure. 
(4) Temperature of the water. 
/ 
(1) Rocks contain pores which, in the absence of a liquid,, 
are ordinarily filled with air. The relative proportion of these 
spaces in the rock to the whole volume is the measure of the poros¬ 
ity. Thus if a cubic foot of sandstone will hold in its pores one- 
fourth cubic foot of water, its porosity is 25 per cent. The greater 
the porosity, the more water absorbed by the rocks. 
(2) The size of the pores in the rock affects the rate of flow. 
Rocks having large pores receive and conduct water many times 
more rapidly than those having small pores. 
(3) The greater the pressure, other conditions remaining the 
same, the more rapid the flow. A pressure of one pound per square 
inch is required to support each 2.31 feet of a column of distilled 
water at the temperature of 60 degrees F. The weight of watei 
from the deep zones is increased by solids in solution and in sus¬ 
pension, and is affected by changes in temperature. Something 
more than a hundred pounds pressure to the square inch is required 
to cause a flow from the bottom of a well 231 feet deep. Something 
more than 500 pounds pressure to the square inch is required to 
cause the rise of water in a boring a distance of 1150 feet. Pres¬ 
sure of this magnitude must materiallv assist in forcing water 
through the rock. 
(4) The temperature of the water is found to influence the 
rate of flow. Slichter finds that a change from 50 to 60 degrees 
F. increases the capacity to transmit water under identical con¬ 
ditions by about 16 per cent.* 
DEPTH OF UNDERGROUND WATER. 
The limit of the downward extent of water has not been reach¬ 
ed by borings or tunnels, some of which exceed a mile in depth. 
Water, while thus known to penetrate to a depth greater than a 
mile, probably does not reach beyond five or six miles at the most. 
The movement, as has been stated, is through natural openings in 
the rock. Pressure increases in the earth with depth, and it is 
estimated that at a depth of approximately six miles, the pressure 
is so great that the pores and cavities of even the strongest rocks, 
are completely closed,t making it impossible for water to penetrate 
*Water Supply and Irrigation Paper, U. S. Geol. Surv. No. 140, p. 13, ,'905. 
fL. M. Hoskins, 16th Ann. Rept. U. S. Geol. Surv., Part I, p. 859, 1896. 
