342 
ton of coal, agrees very well with our figure. This amount is less than 
that given by some other authorities. Andros (’15) says that for “long- 
wall mines” in District 1, from 1.5 to 3 props per ton of coal are needed 
and for “Rooms and Pillar” timbering from 2 to 12 props per ton of coal, 
depending upon roof conditions. 
III. Forest Problems 
FrreEs 
Fire is the most important single factor affecting the forests of this 
region, and fires must be controlled before any policy looking towards the 
securing of a fully stocked forest can be made effective. Some part of 
the continuous timbered bluff area is burned over every year. The fall 
of 1921 was unfavorable for forest fires because of heavy rain at the 
usual fire period, but with the rank vegetation now on the ground the 
spring of 1923 if fairly dry will be a dangerous period. Out of 33 sections 
in different parts of this region examined during the summer of 1921, 
12 had burned over completely or partially within the last three years. 
The forest-fire hazard is quite pronounced for several reasons. Where 
the stands are well stocked with mature trees, the forest floor is carpeted 
with leaves in autumn; and where the stands have been opened up too 
freely grass and weeds grow luxuriantly on the fertile soil. Hence, 
throughout the wooded area conditions favor burning. Also the con- 
tinuity of the wooded areas in the bluffs permits fires to spread if once 
started, and it is almost certain that they will be started somewhere in the 
hills by hunters, by travelers, by settlers clearing land, by tie hackers or 
other woods workers who have no. interest in the woods, by farmers 
burning along the edges of the woods to kill chinch-bugs, by railroad 
engines, by incendiaries, or by lightning. Probably hunters set more fires 
than any other class. Fires also burn into the hills from the Mississippi 
bottoms. 
DIRECT EFFECTS OF FIRES 
The direct damage done by fires (Hawley ’21) consists in the injury 
to trees containing merchantable material, the injury to young growth, 
including reproduction, injury to the soil, and injury to the productive 
power of the forests. A temperature of 54 degrees Centigrade (129.2 F.) 
is sufficient to kill the cambium or growing layer of a tree, this layer being 
very much more sensitive to heat during the early part of the growing sea- 
son. Trees standing upon slopes are usually damaged worst on the uphill 
side, if the fire is sweeping up the slope, because that side is more sheltered 
temporarily and there is usually a greater accumulation of leaves and 
litter there at the base of the tree so that the fire burns longer and more 
severely. 
