224 ECONOMIC GEOLOGY 
on the speed of the river. A go-feet channel at Mildura, on 
the Murray River, which lost 32} inches of water a day 
when the stream was flowing, when the water was stagnant 
lost } inch in the first hour, and after the first day lost only 
I inch per day. Stagnant water deposits clay on its bed 
and renders the channel watertight; a flowing stream keeps 
the clay in suspension and its bed porous. 
The water that percolates underground forms a widespread 
sheet in the pores, crevices, and joints in the rocks; the 
upper surface of this sheet is known as the ** water-table ” 
(Fig. 57). A pit sunk below the water-table is filled with 
water and serves as a well; where the ground falls below 
the water-table in a valley or hill-side, the water outflows as 
a spring. The water-table is an undulating surface, which 
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at 
3 
3 
Fie. 57.—PosiTioN oF THE WATER-TABLE, 
The position of the water-table, WT, in an irregular island. SL = sea 
level. 
repeats the relief of the ground above though the variations 
are gentler; it is at the surface on the shore of a lake or the 
sea ; it rises below hills and falls below valleys. Its varying 
height depends on surface-tension, by which water adheres 
to a surface in a thin film, so that it spreads over particles 
of earth and keeps them wet. Surface-tension and friction 
prevent the water-table becoming horizontal. As surface- 
tension is lessened by heat, a rise of temperature sets water 
free, and thus springs and drains in soils may have an in- 
creased flow after warm weather. | 
Tre CIRCULATION OF SUBTERRANEAN WATER—PIEZO- 
METERS— Lhe passage of water through rocks is subject to the 
laws that regulate its flow through tubes. Tubes which areless 
than 5th of an inch in diameter and spaces less than 134th 
of an inch wide are said to be capillary (or hair-like) and water