WATER SUPPLY OP EASTERN AND SOUTHERN PEORIDA. 133 
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, thread¬ 
ing its way through pores, breaks, crevices, joints and other open¬ 
ings 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 
secondary 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 observa¬ 
tion, 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. 
