28 BULLETIN 1495, U. 8. DEPARTMENT OF AGRICULTURE 
in order but with much lower figures by the grass and woodland group, 
the western yellow pine-mixed conifer, the sugar pine-fir and fir, and 
the Douglas fir. The average for all nontimbered types was 20.9 
per cent and for all timbered types 11.7 per cent. Comparable figures 
for the other national-forest groups point toward similar conclusions. 
High hazards concentrated on a small area are more readily handled 
‘than if widely distributed, and thus the difficulty of meeting extreme 
hazard increases materially from the high-altitude fir to the low-alti- 
tude chaparral. 
SIZE OF FIRES, RATE OF SPREAD, SPEED OF .TTACK 
Speed of attack is one of the dominant factors controlling the final 
size attained by a fire in any locality, despite exceptions caused by 
extreme weather conditions during short periods. It would appear 
possible to sort into groups within each type all fires attacked within 
one hour, within one to two hours, and so on, and then by plotting a 
curve of acres burned over elapsed time to determine directly the 
desired relationship. Even with over 10,000 fires as a basis, however, 
the number of subdivisions to be made and the variability of spread 
in individual fires are so great that a satisfactory curve can not be 
derived in this manner. 
TaBLE 15.—Elapsed time from start of fire to work begun in relation to final size of 
fires, general-risk fires, 1911-1920, 12 tembered forests 
: Index 
Total Total Average | Size of (size of 
Type fires | Clapsed | elapsed | average fire, 
= time time fire elapsed 
time) 
Number | Hours Flours | Acres 
Wiestern-yellowspines=> 25 S58 = = sake Sea ee oes 825 12, 900 15. 64 | 128 8. 18 
Mixed Conifers wes ae Varies knee i ee 658 12, 141 18. 45 70 3. 79 
DD OU STAS fire ene eee ore ee tere a eee eee 76 1, 325 17. 43 143 8. 20 
Sears pineahinsh eek cee eee he ae Ri a Sele 33 702 21. 27 92 4.33 
TEDL T ogee ee eae ea Teg Ce pe pey et eG Sh ta lm ees fy eae 178 4,831 27. 14 29 1.07 
GT ASS es ho ae ae Be eb eer pe ee 57 425 7.46 177 23. 73 
(OE jaye eh gay Ve ae Spee wee ene ba A DW i aR 104 1, 019 9. 80 539 55. 00 
Woo dla nGdiiks sascha ss ee ciao Jo bee tay Meee eee 47 1, 001 21. 30 268 12. 58 
IBTUSHee Sosos neue tat ee eS Eee SEE Nee nN 201 38, 689 18.35 253 13. 79 
Total:OMmaverageescosshaewssesecseeesceeeuebes 2, 179 38, 033 17. 45 135 7. 74 
SUMMARY BY TYPE GROUPS 
Western yellow pine, mixed conifer group__-.-_------ 1, 483 25, 041 16. 89 102 6. 04 
Sugar pine-fir, fir, Douglas fir group-_----...---__---- 287 6, 858 25. 90 66 2.76 
IAI Elm Denese see wns Re eS See sec iMiuMee 1, 770 31, 899 18. 02 96 5. 33 
Chaparral, grass, woodland group_.._--------------- 208 | 2, 445 | 11.75 | 379 32. 26 
As a substitute measure, the acreage of the average general-risk 
fire and the elapsed time from start of fire to work begun on the 
average fire have been computed for each type (Table 15 and fig. 8), 
and from these figures an index has been derived which combines 
two factors, rate of spread and difficulty of control. For example, 
suppose two fires, one in grassland and one in chaparral, are attacked 
by equal crews, each fire being 10 acres in extent at the end of one 
hour. Obviously the rate of spread is the same for both, but the 
