ream has a
still greater speed and strength, and gushes forth with a force
determined by the height of the water above _c_.

[Illustration: FIG. 155.--The flow from an opening depends upon the
height of water above the opening.]

The dam at Johnstown was nearly 1/2 mile wide, and 40 feet high, and
so great was the force and speed of the escaping stream that within an
hour after the break had occurred, the water had traveled a distance
of 18 miles, and had destroyed property to the value of millions of
dollars.

If a reservoir has a depth of 100 feet, the pressure exerted upon each
square foot of its floor is 62.5 x 100, or 6250 pounds; the weight
therefore to be sustained by every square foot of the reservoir floor
is somewhat more than 3 tons, and hence strong foundations are
essential. The outward lateral pressure at a depth of 25 feet would be
only one fourth as great as that on the bottom--hence the strain on
the sides at that depth would be relatively slight, and a less
powerful construction would suffice. But at a depth of 50 feet the
pressure on the sides would be one half that of the floor pressure, or
1-1/2 tons. At a depth of 75 feet, the pressure on the sides would be
three quarters that on the bottom, or 2-1/4 tons. As the bottom of the
reservoir is approached, the pressure against the sides increases, and
more powerful construction becomes necessary.

Small elevated tanks, like those of the windmill, frequently have
heavy iron bands around their lower portion as a protection against
the extra strain.

Before erecting a dam or reservoir, the maximum pressure to be exerted
upon every square inch of surface should be accurately calculated, and
the structure should then be built in such a way that the varying
pressure of the water can be sustained. It is not sufficient that the
bottom be strong; the sides likewise must support their strain, and
hence must be increased in strength with depth. This strengthening of
the walls is seen clearly in the reservoir shown in Figure 152. The
bursting of dams and reservoirs has occasioned the loss of so many
lives, and the destruction of so much property, that some states are
considering the advisability of federal inspection of all such
structures.

[Illustration: FIG. 156.--The lock gates must be strong in order to
withstand the great pressure of the water against them.]

200. The Relation of Forests to the Water Supply. When heavy rains
fall on a bare slope, or