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 when snow melts on a barren hillside, a small amount of the water sinks into the ground, but by far the greater part of it runs off quickly and swells brooks and streams, thus causing floods and freshets.

When, however, rain falls on a wooded slope, the action is reversed; a small portion runs off, while the greater portion sinks into the soft earth.  This is due partly to the fact that the roots of trees by their constant growth keep the soil loose and open, and form channels, as it were, along which the water can easily run.  It is due also to the presence on the ground of decaying leaves and twigs, or humus.  The decaying vegetable matter which covers the forest floor acts more or less as a sponge, and quickly absorbs falling rain and melting snow.  The water which thus passes into the humus and the soil beneath does not remain there, but slowly seeps downward, and finally after weeks and months emerges at a lower level as a stream.  Brooks and springs formed in this way are constant feeders of rivers and lakes.

In regions where the land has been deforested, the rivers run low in season of prolonged drought, because the water which should have slowly seeped through the soil, and then supplied the rivers for weeks and months, ran off from the barren slopes in a few days.

Forests not only lessen the danger of floods, but they conserve our waterways, preventing a dangerous high-water mark in the season of heavy rains and melting snows, and then preventing a shrinkage in dry seasons when the only feeders of the rivers are the underground sources.  In the summer of 1911, prolonged drought in North Carolina lowered the rivers to such an extent that towns dependent upon them suffered greatly.  The city of Charlotte was reduced for a time to a practically empty reservoir; washing and bathing were eliminated, machinery dependent upon water-power and