rs soils earlier in the spring.

12. It prevents the throwing out of grain in winter.

13. It allows us to work sooner after rains.

14. It keeps off the effects of cold weather longer in the fall.

15. It prevents the formation of _acetic_ and other organic acids, which
induce the growth of sorrel and similar weeds.

16. It hastens the decay of vegetable matter, and the finer comminution
of the earthy parts of the soil.

17. It prevents, in a great measure, the evaporation of water, and the
consequent abstraction of heat from the soil.

18. It admits fresh quantities of water from rains, etc., which are
always more or less imbued with the fertilizing gases of the atmosphere,
to be deposited among the absorbent parts of soil, and given up to the
necessities of plants.

19. It prevents the formation of so hard a crust on the surface of the
soil as is customary on heavy lands.

* * * * *

[How does under-draining prevent drought?]

1. Under-draining _prevents drought_, because it gives a better
circulation of air in the soil; (it does so by making it more open).
There is always the same amount of water _in_ and _about_ the surface of
the earth. In winter, there is more in the soil than in summer, while in
summer, that which has been dried out of the soil exists in the
atmosphere in the form of a _vapor_. It is held in the vapory form by
_heat_, which acts as _braces_ to keep it distended. When vapor comes in
contact with substances sufficiently colder than itself, it gives up its
heat--thus losing its braces--contracts, and becomes liquid water.

This may be observed in hundreds of common operations.

[Why is there less water in the soil in summer than in winter,
and where does it exist?

What holds it in its vapory form?

How is it affected by cold substances?

Describe the deposit of moisture on the outside of a pitcher in summer.

What other instances of the same action can be named?]

It is well known that a cold pitcher in summer robs the vapor in the
atmosphere of its heat, and causes it to be deposited on its own
surface. It looks as though the pitcher were _sweating_, but the water
all comes from the atmosphere, not, of course, through the sides of the
pitcher.

If we breathe on a knife-blade, it condenses in the same manner the
moisture of the breath, and becomes covered with a film of water.

Stone houses are damp in summer, because the inner surfaces of the