n or snow, or to form dews. This is sometimes called the fly-off,
and except for some changes caused by management of the land, is
entirely beyond control.

A part of the remainder sinks into the soil below the surface. A large
portion of this helps to cause the slow rock-decay that forms the soil,
and which is known as ground water. It is estimated that within the
first hundred feet below the surface of the earth there is a quantity of
water that has seeped down; and that would form, if it were collected, a
vast reservoir sixteen or seventeen feet in depth spreading over all the
3,000,000 square miles of the area of our country. This is equal to
about seven years' rainfall and is a very important part of our water
resources. In many places it forms into underground streams or lakes. It
feeds all the springs and many of the lakes. Our wells are dug or
drilled into this underground water system. It carries away the excess
of salts and mineral matter from the soil, the trees strike their roots
deep into the earth and draw from it, and last and most important of
all, that which sinks immediately below the surface supplies all our
plant growth. So that it is this last portion, that which sinks below
the ground, and which is sometimes termed the cut-off, amounting to
about one-tenth of all our water resource, or about the quantity that
flows through the Mississippi River system, that forms the really
important part.

On this depends all that makes a land habitable, the water for drinking
purposes and for plant and animal growth. On it depends the rate of
production of every acre of farm and forest land and the life of every
animal. Every full-grown man of one hundred and fifty pounds takes into
his system not less than a ton of water each year, and every bushel of
corn requires for its making fifteen or twenty tons of water.

Of the importance of this Professor Chamberlain says: "The key to the
problem of soil conservation lies in due control of the water that falls
on every acre. This water is an asset of great value. It should be
counted by every land owner as a possible value, saved if turned where
it will do good, lost if permitted to run away, doubly lost if it also
carries away the soil and does destructive work below."

The uses of rainfall are given thus:

A due portion should go through the soil to its bottom to promote rock
decay. Some of it should go into the underdrainage to carry away harmful
matter, anothe