252 



WOOD AND FOREST. 



is particularly liable to burn again, on account of the accumulation 

 of dry trunks and branches. See Fig. 107. 



Long dry seasons and high wind furnish particularly favorable 

 conditions for fire. On the other hand, the wind by changing in di- 

 rection may extinguish the fire by turning it back upon its track. 

 Indeed the destructive power of fires depends largely upon the wind. 



Causes of fire. Forest fires are due to all sorts of causes, accidental 

 and intentional. Dropped matches, smouldering tobacco, neglected 



Fig-. 105. Forest Fire. U. S. Forest Service. 



camp fires and brush fires, locomotive sparks, may all be accidental 

 causes that under favorable conditions entail tremendous loss. There 

 is good reason to believe that many forest fires are set intentionally. 

 The fact that grass and berry bushes will soon spring up after a fire, 

 leads sheep men, cattle and pig owners, and berry pickers to set fires. 

 Vast areas are annually burned over in the United States for these 

 reasons. Most fires run only along the surface of the ground, doing 

 little harm to the big timber, and if left alone will even go out of 

 themselves ; but if the duff is dry, the fire may smoulder in it a long 

 time, ready to break out into flame when it reaches good fuel or 



