<J78 JOURNAL OF FORESTRY 



interesting and instructive to remember that wind pressure based on 

 velocity also varies as the square of the velocity. 



It must be perfectly evident that the data here used are insufficient 

 to justify the definite and final statement that the above law actually 

 expresses the relation. Considering, however, the care with which the 

 observations were made and the rather close agreement with the 

 formula of the empirically derived values, the law may at least serve as 

 a working hypothesis, against which future data may well be checked. 



With the data presented, it may be of interest to compute the prob- 

 able relative rates of spread of fires at different hours of the day, as 

 governed by wind velocity. 



Table 8 



Hour \ 



12 Mt. 



3 A.M. 



6 A.M. 



9 A.M. 

 12 M. 



3 P.M. 



6 P.M. 



9 P.M. 



Velocity data from Chart 6. 



Thus, the lower temperature and higher humidity, both of the air 

 and of the litter, being left out of consideration, fires, as influenced by 

 wind, are likely to spread only half as fast at night as during the early 

 afternoon. 



SUMMARY 



This preliminary study shows : 



1. Litter is capable of holding its own weight of moisture, but a 

 layer of litter from 1 to 11^ inches deep can, under normal conditions 

 of summer weather, be reduced from the saturation point to air dry 

 in from one to two days. 



2. Litter with over 8 per cent of moisture will not burn, and this 

 is therefore the critical point. 



3. During the fire season great fluctuations in moisture content 

 occur on different exposures and at different elevations. On a north 

 slope the moisture content may be above the danger point for three- 

 fifths of the season as compared with one-third on a south slope during 

 the same period 



