694 
Journal o f Agricultural Research 
Vol. XXX, No. 8 
will usually be necessary to suppress 
fires to keep them from running over 
on lands with greater liability, or be¬ 
cause of the consequent effect on the 
fire hazard in general. 
The principle may be illustrated by 
a diagram (fig. 1), which shows the 
curve for loss plus suppression costs 
(X-Y) descending as the line represent¬ 
ing primary protection costs (A-B) 
rises, while the curve S-T, represent¬ 
ing the sum of the two, falls to a point 
P, then rises steadily. The expendi¬ 
ture at which P is attained, or E, rep¬ 
resents the proper amount to spend 
for primary protection. A greater 
expenditure might indeed reduce the 
loss and suppression cost, but not 
sufficiently to reduce the total, and so 
might not be justified. 
The purpose of this study, then, was 
to determine whether it is possible to 
rate the liability of loss as well as the 
probable cost of suppression, which 
together may be termed the total 
liability. No attempt was made to 
actually rate the fire hazard and 
liability for specific forest units, but 
principles and methods have been 
worked out as a basis for a detailed 
field survey or “fire reconnaissance,” 
which must necessarily lie at the 
foundation of any rating for specific 
forest units. 
FACTORS OF HAZARD AND LIABILITY 
The probability of loss is governed 
primarily by the values of destructible 
resources, and by the hazard, or chance 
of their destruction as a result of ex¬ 
posure to fire. Values of forest re¬ 
sources may be classified under the 
following heads: (1) Timber, includ¬ 
ing mature timber, young growth, and 
the forest capital, which includes soil 
productivity; (2) forage; (3) indi¬ 
rect values, including watershed pro¬ 
tection (regulation of streamflow and 
prevention of erosion and floods) and 
occupancy values, such as recreational 
use, improvements, game resources, 
and the like. 
FIRE HAZARD 
The chance of destruction by fire 
of the values on a given forest area 
depends upon the probability of its 
being burned over, and upon the prob¬ 
ability that the values will be de¬ 
stroyed as a result of such burning. 
Its chance of being burned over depends 
upon whether fires will start on or near 
it, and upon the area that such fires 
will cover. 
Whether or not fires will start 
depends upon the presence or absence 
of causative agencies during the period 
in which fires can start. These agencies 
may be classified as follows: 
Human agencies: Campers (includ¬ 
ing campfires and fires caused by 
smokers and hunters), lumbering opera¬ 
tions, railroads, brush burning, in¬ 
cendiaries, and miscellaneous. 
Natural agencies: Lightning. 
The area that will be burned over 
depends upon a large number of factors 
and subfactors, which may be outlined 
as follows: 
1 . Inflammability determines rate of spread, and 
depends upon the character of: 
a. Cover, including timber, undergrowth, and 
litter, all of which furnish the fuel for fires. Inflam¬ 
mability of timber depends upon the species, age, 
density, and uniformity of the stand; and the condi¬ 
tion of the stand, including such points as the 
presence of catfaces, moss on the trunks and lower 
branches, and standing dead snags. Inflammability 
of the undergrowth depends upon its character 
(grass, weeds, brush, or tree reproduction), amount 
(density and height), and uniformity of distribu¬ 
tion. Inflammability of the litter is determined by 
its character (duff, dead grass and herbage, needles, 
twigs, cones, branches, logging slash, windfalls), 
its amount, and its condition as to dryness, decay, 
compactness, etc. 
b. Climate and weather, which not only have 
much to do with determining the character and 
condition of the cover, but also influence directly 
the action of fires. The important climatic factors 
are: (1) Precipitation, both annual and seasonal, 
especially its amount and distribution during the 
dry seasons; (2) temperatures, means and maxima, 
especially during dry parts of the year; (3) humidity, 
including fogs, dews, etc., during the dry seasons; 
(4) evaporation, affecting the rate of drying of 
inflammable material; (5) soil moisture; (6) wind 
direction and velocity during the dry season. 
c. Topography, which with climate practically 
determines the character of cover, and also directly 
affects the spread of fires, by the degree of slope, by 
the aspect, by the uniformity of terrain, and by the 
absolute and relative altitude, which influence 
atmospheric factors. 
2 . Controllability determines whether fires can be 
extinguished while small, or whether they will burn 
over large areas. It depends upon: 
a. Men and equipment available to fight fires. 
b. Accessibility—the time required to detect and 
to reach a fire, together with the routes and possible 
methods of travel. 
c. Topography and soil, which influence the speed 
and cost of control work, such as trenching. Natu¬ 
ral breaks, such as cliffs, streams, and bare ridges, 
and artificial breaks, such as roads and fire lines, 
should be considered here, as may also the avail¬ 
ability of water for use on the fire line. 
d. Type of forest and ground cover, which influ¬ 
ence the method of attack, as well as the speed and 
cost of the work. 
e Degree of efficiency with which suppression 
work is carried on. 
Even though a given area of forest 
may burn over, it does not necessarily 
follow that all or even a major part of 
the values on the burned area will be 
destroyed. The chance of destruc¬ 
tion, which may be called the loss 
ratio, or the destructibilitv, depends 
upon the susceotibilitv of the various 
resources to direct and indirect fire 
damage, and also upon the intensity of 
the fire. These in turn depend upon 
