102 
FLORIDA GEOLOGICAL SURVEY—THIRD ANNUAL REPORT. 
leaves of plants, and that not more than one-half of the total 
rainfall in Florida passes through the soil and surface material 
to join the underground water supply. 
AMOUNT OF WATER AVAILABLE FOR THE UNDERGROUND 
SUPPLY. 
An annual rainfall of 53 inches is found by computation to 
amount to 921,073,379 gallons per square mile. Of this amount 
it is estimated that in central Florida about one-half is added each 
year to the underground water supply. 
UNDERGROUND CIRCULATION OF WATER. 
Underground water is found usually to be in motion, threading 
its way through pores, breaks, crevices, joints and other openings 
in the rocks. Its movement is ordinarily slow and varies with 
different rocks and under different conditions. 
CAUSE OF MOVEMENT. 
The chief cause of movement of underground, as of surface 
water, is gravity. Capillarity is an additional force which under 
special conditions, may become the controlling factor. The water 
returned to, and evaporated from the surface of the ground, as 
well as that carried to and evaporated from the leaves of plants, 
is moved by capillarity in opposition to gravity. Gravity, how¬ 
ever, is the controlling force in the movement of water through 
the deep zones of the earth. Pressure, which is an important sec¬ 
ondary cause of the movement in the earth, is the expression of 
gravity. Except in the case of capillarity, the movement of water 
apparently in opposition to gravity, is, upon closer observation, 
found to be in reality, movement in response to gravity. The water 
which rises in a boring or flows from an artesian well or spring 
is forced up by pressure due principally to the weight of water 
lying at a higher level. The familiar observation that water seeks 
its own level has the same explanation. 
RATE OF MOVEMENT. 
The chief factors affecting the rate of movement of water 
through a porous medium as given by Slichter are as follows :* 
*Water Supply Paper, U. S. Geol. Surv., No. 67, p. 17, 18, 1902. 
