In the centre of the lines a narrow trench is dug to mineral soil or the lines are plowed or burned over so that they are bare of fuel.
Such lines also are of value around woods and grain fields to keep the fire out.

They are commonly used along railroad tracks where locomotive sparks are a constant source of fire dangers.
Our forests, on account of their great size and the relatively small man force which guards them, are more exposed to fire dangers than any other woodlands in the world.

The scant rainfall of many of the western states where great unbroken areas of forest are located increases the fire damages.

The fact that the western country in many sections is sparsely settled favors destruction by forest fires.

The prevalence of lightning in the mountains during the summer adds farther to the danger.