INJURY CAUSED BY FOREST FIRES 253 



At first a surface fire burns out from its starting point in 

 circular form, and would so continue were it not for various 

 factors which at once begin to control its development. The 

 wind is of primary importance and soon gives the fire an 

 elliptical shape, the leeward side progressing most rapidly 

 while the windward side burns slowly or dies out. 



Crown fires advance only in the direction of the wind. 

 Burning brands and embers are carried forward by the wind 

 and set other fires in advance of the original one. Thus 

 several fires both surface and crown are apt to be burning 

 and may combine. Carried by the wind a crown fire may 

 leap a quarter of a mile or more from one side of a valley to 

 the other, sometimes leaving unburned the timber in the 

 center. 



Ground fires progress slowly, not being affected by the 

 wind, with rather uniform destructiveness and consume all 

 the inflammable material in their path. Such a fire may 

 bum over several acres in a day or may cover only a fraction 

 of an acre. 



Surface fires range from a slow advance in a forward direc- 

 tion of less than a mile per day to an average of several miles 

 per day and in the case of some grass and brush fires may attain 

 a speed of several miles per hour. 



Crown fires progress with great rapidity, in fact speed is 

 essential for the existence of a crown fire. It is more difficult 

 to^secure definite figures for the spread of such fires as con- 

 trasted to the other two classes. Graves ^ states that six to 

 seven miles per hour is probably a maximum rate. 



In the past the most severe fires have occurred in the 

 northern part of the United States above the 43rd parallel of 

 latitude in three districts: (i) New York and the New 

 England States, (2) the Lake States and (3) the Pacific 

 northwest including Western Montana, Idaho, Washington 



