60 PHYSICAL GEOGRAPHY.

 

the waters that sink into the ground collect.
Reservoirs are sometimes large subterranean
basins, but more frequently are merely porous
strata, such as beds of sand or gravel, whieh lie
between impervious layers of clay or hard rock.
The water collects in the spaces between the par-
ticles of sand or gravel.

159. Size of Reservoir—When the reservoir
is large, the spring is constant; when smail, the
spring is temporary.

Constant Springs are those which flow continu-
ally, and are but little affected in the volume of
their discharge even by long-continued droughts.

Temporary Springs are those which flow only
for a short time after wet weather, drying up on
the appearance of even moderate droughts.

The quantity of water discharged by a spring depends on the
size of the orifice or outlet tube, and the depth of the outlet be-
low the surface of the water in the reservoir. The flow is
proportional to the square root of the depth. That is to
say, if with a given depth of orifice the velocity be one
foot per second, in order to make the water escape with
twice the velocity the depth must be increased fourfold.

The actual velocity is somewhat less than this, being di-
minished by friction.

Since the volume discharged by some springs
is very considerable, we must infer that their
reservoirs are of great size. Many springs prob-
ably receive the drainage from hundreds of
square miles of surface.

160. Shape of the Reservoir—When the out-
let tube of the reservoir is siphon-shaped, the dis-
charge of the spring becomes periodical. The

    

SY SS

  

Fig. 58 A Periodical Spring.

spring continues to discharge its waters for a
time, and then stops flowing, even during wet
weather. After a certain interval it again dis-

 

 

charges. The times during which the spring con-
tinues to discharge are always practically the
same. Hence the spring is called a periodical

spring.

The cause of periodical springs is due to the siphon-
shape of the outlet tube. A siphon is a tube so bent as to
have two vertical arms of unequal length. When filled,
it will continue to discharge as long as its shorter arm is
below the water and the longer arm free. If a large cav-
ernous reservoir be in connection with the surface of the
earth by a tube of this shape, it will begin to discharge its
water when, by infiltration, the level reaches the highest
bend of the tube, as at a, in Fig. 58, since the water will then
drive out the air and fill the entire tube. The discharge
will then continue until the water-level falls below the
mouth of the tube, or at 0, in the figure. The time of the
discharge is always practically the same, since the same
quantity is discharged each time under exactly similar
conditions. :

Springs are common on the shores of the ocean. Their
waters are fresh because the outflow of the fresh water
prevents the inflow of the salt water. This is the case
even on coral islands, where the height of the land is
but ten or twelve feet above the sea. A comparatively
shallow well, on such islands, generally yields fresh water,
derived, of course, from the rainfall.

161. Depth of Reservoir—According to the
distance the reservoir is situated below the sur-
face of the earth, springs are divided into Cold,
and Hot or Thermal.

Cold Springs are those whose temperature does
not exceed 60° Fahr. Their waters are sometimes
much colder than 60° Fahr.

Very cold springs owe their low temperatures
to the sources whence they draw their supplies.
In mountainous districts these can generally be
traced to the melting of huge snow-fields, or
masses of ice called glaciers. The temperature
in such cases is often nearly that of ordinary ice-
water.

The reservoirs of all springs the temperature
of whose waters ranges from 50° to 60° are, in
general, comparatively near the surface. They
are colder than surface waters—

(1.) Because they are shielded from the sun ;

(2.) Because evaporation occurs in their cav-
ernous reservoirs. ;

The temperature of springs of this kind is, in
general, but slightly affected by changes in the
temperature of the outer air. Since the reservoirs —
of ordinary springs are shielded from the hot air
in summer and from the cold air in winter, their
waters are colder than river-water in summer, and
warmer than river-water in winter. Their waters
average, in their temperature, that of the strata
over which they flow in their subterranean course.