230. ECONOMIC GEOLOGY 
dangerous. These wells are shafts sunk into a porous for- 
mation, into which sewage is discharged and drains away. 
Such wells are useful in flat lowlands where there is no 
surface slope for the drainage. A legal decision in 1885 
prohibits the use of such wells where they may contaminate 
the underground water supply One town is not allowed to 
pour its sewerage into the bed from which another town 
draws its drinking water; but the owner of any plot of land 
has the right, under both British and American law, to draw 
from it as much water as he can. 
WeLLs AND SpriNGs—The simplest condition for a spring 
or well is where a porous bed, such as sand, rests on an im- 
permeable bed. Rain-water percolates into the sand until 
the water-table rises sufficiently above an outlet to discharge 
there as a spring. Lines of springs occur along hill-sides 
where wet sand rests on clay. If an excavation be made at 
such a position water will flow into it, and it forms a shallow 
well or soak. The traverse of desert countries often depends 
on the discovery of the right positions at which to dig soaks. 
They are often beside pools of salt water which remain a 
foot or so deep where there has been no rain for many 
months, and evaporation removes 10 feet a year. These 
pools must be renewed by soakage from adjacent beds. 
The positions of discharge of the water may be indicated 
by a few rushes or microscopic alge on the ground, or on a 
still day by a slight tremor in the air due to the different 
refractive index of the moist air.- Where the ground is 
charged with salt the soak will only yield salt water, and a 
position has to be found where a soak will occur above the 
salt-charged level. 
Soaks give but small yields; for they only occur where 
the water percolates slowly or the supply would run away 
quickly, instead of oozing out for months or years. Most 
wells are similar in principle to soaks, though the water- 
bearing layer is larger, and contains more water, and the yield 
is larger and quicker. 
FLowine WeLLs—Prolific deep wells often occur (Fig. 59) 
where a permeable bed passes underground between two 
impermeable beds. The water that percolates from the 
outcrop is at length prevented from sinking deeper by the 
thinning out of the porous bed, or its becoming compact