226 ECONOMIC GEOLOGY 
The height to which water rises in rocks may be illustrated 
by a series of ‘* piezometers™ or pressure meters, as in 
Fig. 58. If a horizontal pipe from the lower part of a water 
tank is closed at the outer end by the tap T and bears a row 
of vertical tubes (P1, P2, P3, P4), the water will rise in each 
of them to its height in the tank, and that level is the hydro- 
static surface (HsS). If the tap be opened the water in 
the vertical tubes falls to a surface sloping from the water- 
level in the tank to the outlet. As the water is flowing, 
the conditions are hydraulic, and the inclined surface, HI Si, 
is the hydraulic slope or gradient. This gradient depends 
ba) 
-Hs S 
ase. 
~ 
F16. 58.— WATER-LEVEL, 
Water-level in a series of piezometers P,-P,. HsS = Hydrostatic Surface. 
HI SI = Hydraulic Gradient. 
on the velocity of the water along the pipe; the greater 
its velocity, the lower the water-level in the vertical pipes. 
The height of the water in each of them is the pressure-head ; 
the difference of pressure-head between flowing and stagnant 
water is the ‘‘velocity-head’; the pressure-head and 
velocity-head together equal the hydrostatic-head. If the 
horizontal pipe be constricted (Fig. 58 B) so that the flow 
of water along it be reduced, or if part of it be filled with 
sand so that the friction is increased, then the pressure-head 
is raised. The hydraulic gradient therefore varies with the 
conditions in the outlet channel, and may be an irregular 
slope. If the pipes and the side of the tank were replaced 
by a block of porous sandstone the water would soak into 
the stone and flow through it to an outlet at the free end under