h the trees need for
their sustenance. They progress slowly and kill or weaken the roots
of the trees.

[Illustration: FIG. 133.--A Top Fire near Bear Canyon, Arizona.]

Top fires, Fig. 133, are the most dangerous, destroying everything
in their way. They generally develop from surface fires, though
sometimes they are started by lightning. They are more common in
coniferous forests, because the leaves of hardwoods do not burn so
readily. Checking the progress of a top fire is a difficult matter.
Some fires will travel as rapidly as five miles an hour, and the
heat is terrific. The only salvation for the forest lies, in many
cases, in a sudden downpour of rain, a change of wind, or some
barrier which the fire cannot pass. A barrier of this kind is often
made by starting another fire some distance ahead of the principal
one, so that when the two fires meet, they will die out for want of
fuel. In well-kept forests, strips or lanes, free from inflammable
material, are often purposely made through the forest area to
furnish protection against top fires. Carefully managed forests are
also patrolled during the dry season so that fires may be detected
and attacked in their first stages. Look-out stations, watch-towers,
telephone-connections and signal stations are other means frequently
resorted to for fire protection and control. Notices warning campers
and trespassers against starting fires are commonly posted in such
forests. (Fig. 143.)

[Illustration: FIG. 134.--Sheep Grazing on Holy Cross National Forest,
Colorado. The drove consists of 1600 sheep, of which only part are shown
in the photograph.]

The grazing of sheep, goats and cattle in the forest is another
important source of injury to which foresters must give attention.
In the West this is quite a problem, for, when many thousands of
these animals pass through a forest (Fig. 134), there is often very
little young growth left and the future reproduction of the forest
is severely retarded. Grazing on our National Forests is regulated
by the Government.

As a means of protection against insects and fungi, all trees
infested are removed as soon as observed and in advance of all
others, whenever a lumbering operation is undertaken.

[Illustration: FIG. 135.--A Typical Montana Sawmill.]

How forests are harvested: Forestry and forest pres