960 JOURNAL 01" FORESTRY 



Table l 



[Weights are all in grammes.] 



Sample Air-dry Saturated Moisture Percentage of 



number. weight. weight. loss. mosture. 



1 73.650 134.300 60.650 82. 



2 83.950 148.900 64.950 77. 



3 . 70.000 114.800 44.800 64. 



4 81.700 116.350 34.650 43. 



5 70.960 154.660 73.700 104. 



6 74.500 156.760 82.260 110.5 



7 86.620 174.315 87.695 101. 



8 82.450 157.450 75.000 91. 



Average 77.960 144.695 66.735 85.7 



The ignition point was determined experimentally l)y moimting 

 two-thirds of a square foot of undisturbed litter on sheets of tin, air- 

 drying the samples to a constant weight, saturating, and drying in shade 

 until the litter burned freely. The litter was dried slowly so that the 

 moisture loss should be uniform throughout the sample, and weight 

 determinations were made before each test for burning, since, of 

 course, after burning it would be impossible to obtain the moisture 

 content. The data are shown in the table : 



Table 2 



Sample Air-dry Weight when Weight of Percentage of Remarks, 



number. weight. burned. moisture, moisture. 



1 236.370 268.288 30.918 13.5 Failed to burn. 



2 296.000 319.099 23.088 7.8 Burned 



3 343.550 371.726 28.176 8.2 Burned 



4 241.670 263.781 21.511 8.9 Failed to burn. 



It is clear that about 8 per cent of moisture is the critical point. 



Tests made at the Priest River Experiment Station also show that 

 8 per cent, based on oven-dry weight, is the critical point in that region. 

 It has been found by a series of tests here that the air-dry weight of 

 litter at 80° to 90° Fahrenheit, and 20 per cent relative humidity is 

 only 1 to 2 per cent greater than the oven-dry weight. 



The potential fire season then begins as soon as the litter contains 

 8 per cent or less of moisture, or strictly speaking, when the top 

 layers contain that amount, since fire can spread if only the top half- 

 inch or so of a 3-inch layer is dry. 



SEASONAL CHANGES IN MOISTURE CONTEXT OF LITTER 



The moisture content of litter during the fire season, 1916, at 4,000, 

 5,000, 6,000, and 7,000 feet elevations on north and south slopes near 



