Probably the most difficult factors for the Forest Supervisor to appraise on each Ranger district are the fire liability and the fire hazard. Fire liability has to do with the amount of damage a fire could do if it got started. Valuable timber needs protection most of all, and the value of the forest is determined by the kind of trees in it and the density of the stand. Fire hazard is usually expressed in terms of risk. The Supervisor asks his Ranger if the risk on a certain area in his district is high, low, or medium. Risk depends, of course, largely upon the character and inflammability of the forest cover and the presence of human causes. Dense forests involve greater risk than open, scattering trees; government forests interspersed with private holdings containing much old slash have a high risk factor; and government forests near sawmills, large towns, and along railroad rights-of-way also have high risk factors. All these matters must be considered, in order that each area on each Ranger district gets just enough money for fire protection and not a bit more. [Illustration: Figure 39. Putting out a ground fire. Even if the fire does not burn the standing timber, it kills the young trees and so weakens the larger ones that they are easily blown over. Wallowa National Forest, Oregon.] [Illustration: Figure 40. Forest officers ready to leave a tool box for a forest fire in the vicinity. Such tool boxes as these are stationed at convenient places on National Forests ready for any emergency. Arapaho National Forest, Colorado.]