PHYSICAL CONTROLS OF FIRES 923 



Class of fire Per cent in brush Per cent in timber 



A 22 47 



B 36 35 



C 42 18 



Under both sets of conditions the percentage of B fires was the 

 same, but on the average there is over twice as high a percentage of 

 C fires in brush as in timber and. conversely, the percentage of A 

 fires is twice as high in timber as in brush. 



Years ago Supervisor Wynne worked out comparative rates of 

 spread for fires in brush and in timber and found that, on the aver- 

 age, brush fires on slopes up to 40 per cent and wind velocity up to 

 8 miles per hour spread 51 acres per hour, while timber fires under the 

 same range of conditions spread 24 acres, or something less than half 

 as much. These two entirely independent lines of investigation give 

 a ratio of about two to one as representing differences between brush 

 and timber fires. 



It will be seen at once that one very important reason for the high 

 percentage of class C fires on south slopes is in the greater percentage 

 of brush on such slopes, as compared with north slopes. It has been 

 impracticable to tabulate the data to a point where percentage of C 

 fires in timber and brush on the various aspects can be determined, 

 but there can be no question that character of cover as well as cli- 

 matic conditions makes the south slope the most dangerous with w'hich 

 we have to deal. 



It seems reasonable to suppose that the cause for the occurrence of 

 brushfields on south slopes to a greater degree than on others lies 

 in the fact that weather conditions are more severe there than else- 

 where. Once brushfields are established, they themselves tend to 

 continue the difficulty of control of fires and perhaps accentuate the 

 differences between north and south slopes. 



Further work, not only on the relative spread of timber and brush 

 fires but of fires in all the cover types, will be necessary to elaborate 

 and complete the study here reported on. 



SUMMARY 



The study of the influence of degree and direction of slope and of 

 type of cover on rate of spread of fires in California shows : 



1. That on south slopes the per cent of class C fires is about twice 

 as great as- on north slopes and three and one-half times as great as 

 on level land. 



