PROTECTION OF FORESTS FROM FIRE 



511 



ping ledges, and so on, tend to check a 

 fire and to prevent its gathering volume. 

 On extensive level ground, fires burn 

 more uniformly, gather a greater vol- 

 ume, generally do more damage, and 

 extend over a larger area than in rugged 

 topography. 



Character of the Soil 



Any influence which tends to drynt-ss 

 increases the intensity of a fire. Thus 

 on sand and limestone soils, which warm 

 up and dry out readily, fires are likely to 

 be very severe. Southern and western 

 slopes are apt to be more severely 

 burned than others, because they are the 

 warm and dry exposures. . _ 



Condition of the Atmosphere 



The character of a fire is influenced, 

 further, by the condition of the at- 

 mosphere. Roughly, the greater the ve- 

 locity of the wind, the more rapid is the 

 progress of the fire. A fire burns more 

 severely when the wind is constant than 

 when it is gusty. It is the steady high 

 wind which makes the most intense fire. 



A fire burns most fiercely when the 

 atmosphere is dry. Fires are, therefore, 

 most severe during the hot part of the 

 day and when fanned by a dry wind. 

 A moist atmosphere retards a forest 

 fire. The well-known fact that the night 

 is the best time to fight a fire is thus 

 explained ; for at night there is usually 

 little or no wind, while the air is com- 

 paratively heavy and damp. 



Rapidity of Surface Fires 



No reliable estimate of the rapidity of 

 surface fires can be made, because it 

 varies so greatly under different con- 

 ditions. In the hardwood regions of 

 the east a surface fire seldom travels 

 more than five miles a day, but in the 

 coniferous forests of the west instances 

 are known where this rate of speed has 

 been more than doubled. 



Grass Fires 



In nearly all open forests there is a 

 certain quantity of grass which, when 

 dried, carries fire very rapidly. In many 

 forests the presence of grass consti- 

 tutes one of the important problems 

 connected with surface fires. This is 

 particularly true in the southern pine 

 forests. 



A grass fire is more influenced by the 

 density of the grass than by its height. 

 Where the grass is in separated patches, 

 with no leaves or other inflammable 

 material between, it is difficult for a 

 fire to spread. Uniformly dense grass 

 burns with the greatest intensity. High 

 grass burns with greater intensity than 

 low grass, but the fire does not usually 

 run so rapidly. Grass a foot high, if 

 dense, may produce such a hot fire as to 

 start a crown fire. In short grass, with 

 an ordinary wind, a fire will run from 

 three to four miles an hour ; with a 

 high wind, twice as fast. The chief 

 factors affecting the burning of grass 

 are its dryness and the force of the 

 wind. Other factors have their influ- 

 ence, however, just as they do in the 

 burning of litter. 



Brush Fires 



Bushes and small trees frequently re- 

 tain many dried leaves late into the fall, 

 and in some cases even into the follow- 

 ing spring. This is particularly true of 

 some of the oaks. A fire will sometimes 

 run through such brush and do an im- 

 mense amount of damage. Such a fire 

 is called a brush fire. It is carried along 

 in part by the burning of the litter, but, 

 wherever tlie opportunity offers, it runs 

 up through the dried leaves remaining 

 on the brush. In the eastern United 

 States a brush fire is most likely to run 

 during the late fall. Under ordinary 

 circumstances, it has rather the char- 

 acter of a surface fire than that of a 

 crown fire. 



Fires running through young stands 

 of conifers consume the foliage and 

 readily kill the trees. In a very young 

 stand, in which the trees stand isolated 

 and the crowns have not .yet grown to- 



