255. Common springs are evidently merely the outlets
of natural reservoirs of water which exist in every part or
the earth, and are filled by the water whi<?h falls upon the
surface in rain and snow, and gradually percolates through
the soil. When these reservoirs are near the surface, the
supply of water sometimes ceases during long-continued
droughts, and the springs of course become dry ; but
they are often situated so deep in the hills that no tem-
porary cause of this kind can aflect them, and they con-
tinue to flow at all times alike.
But, if the aperture or channel through which tht
water of the reservoir discharges itself, in some part of
its course rises considerably above the bottom of the res-
ervoir, a natural syphon may be formed, which will cause
the spring constituting its outlet to exhibit an intermit-
tent character.
Let A F be a section of
part of a mountain con-
taining a cavern, C, deeply-
seated in it, and having an
aperture or channel, D E
F, leading from it to the
valley or plaj^^t its base.
