134 
FLORIDA STATE GEOLOGICAL SURVEY. 
(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 
porosity. 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 water from the deep zones is increased by solids in solution 
and in suspension, 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 1,150 feet. Pressure of this magnitude must materially 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 condi¬ 
tions, by about 16 per cent.f 
DEPTH OF UNDERGROUND WATER. 
The limit of the downward extent of wafer has not been 
reached by borings or tunnels, some of which exceed a mile in 
fWater Supply and Irrigation Paper, U. S. Geol. Surv. No. 140, p. 13* 
1905. 
