918 JOURNAL OF FORESTRY 



As a criterion of rate of spread, tlie percentage of "C" fires 

 (those over ten acres in extent,) and the size of average fire have been 

 used. '^I'he percentage of class "C" fires is an excellent index of difi^er- 

 ences in rate qf spread on the various slopes and aspects. It is easy 

 to see why this is so. On south slopes, for example, with very dry 

 conditions fires will naturally spread more rapidly than on north slopes 

 where the litter is more likely to be moist than on south slopes, and this 

 greater rate of spread will be reflected in the percentage of fires ex- 

 ceeding a given arbitrary limit of ten acres. For size of average 

 fires the same holds true, the size being controlled both by percentage, 

 of "C" fires and the average size of "C" fires, which latter varies in 

 much the same way with slope and aspect that other criteria and rate 

 of spread do. 



RELATION OF R.\TE OF SPREAD TO ASPECT AND SLOPE 



Table No. 1 shows percentage of class "C" fires on slopes of differ- 

 ent degree and of different aspect. It is to be noted that the percentage 

 of "C" fires increases quite rapidly as the slopes become steeper, the 

 graphic relation approaching a straight line in form. The values on 

 level land are the lowest, as is to be expected. The differences between 

 north and south slopes are particularly striking, while east and west 

 slopes keep an intermediate position and in most cases are fairly close 

 together. 



The percentage of ''B" fires (one-fourth to 10 acres) is practically 

 constant for all slopes and aspects, averaging 35 per cent (range 33 to 

 37 per cent) so that percentage of "A" fires (0 to one-fourth acre) 

 is a reciprocal of percentage of "C"' fires. 



It is to be noted that on the average there are 38 per cent class "C" 

 fires on south slopes as against 21 per cent on north slopes and 1 1 per 

 cent on level land. 



Table 2, based on the sar.e fires as previously used, shows size of 

 average fire on degree and direction of slope. As in the case of per- 

 centage of "C fires, the aspects rank : Level, North, East, West. South. 

 For each aspect the average fire increases directly with percentage of 

 slope, the rates being: Increase in acres per 10 per cent of slope — 

 North, 16 acres; South, GO acres; East, 33 acres: West, 45 acres. 



It is interesting to note that of the total acreage burned (Table 2) 

 56 per cent is due to south slope fires, only 6 per cent to those on level 

 land, while north, east, and west slopes are about equal. 



