418 ' riRE-CONSBRVANCY. 



pends on a great many combined circumstances, the principal of 

 ■which are : — 



(1) Density and height of the grass. The presence of grass is 

 the greatest obstacle there exists to the success of fire cffnservancy., 

 If the grass in a forest could be suppressed or removed, the only- 

 danger left would be that of a pure leaf-fire, and such a fire would 

 be comparatively light, would be a standing menace to the forest 

 during a much shorter time of the year, and, except on a steep 

 slope, would be extremely easy to circumscribe and extinguish. 

 In a forest fire the density of the crop of grass is of fer greater 

 consequence than its mere height, Where it grows in isolated 

 patches, the grass generally burns with difficulty, unless there is a 

 strong and steady breeze blowing. This is especially the case if th© 

 grass is not yet quite dry ; and, under any circumstances, the flames 

 flare up so irregularly along the advancing edge of the conflagration 

 that the fire can usually be beaten out with ease. A thick crop of 

 grass, although visibly green, will often burn steadily and fiercely 

 along the whole line before a very slight breeze, a,nd the fire may 

 even defy any direct attempt to get it imder. On the height of the 

 grass depends, besides the total quantity of ■^ combustible matter 

 present, also the force of the draught of air rising from below up- 

 wards. It hence influences directly the length of the tongues of 

 flame and the height to which they can rise, and, but in an inverse 

 ratio, also the rate of progress of the conflagration, so that if the fire 

 becomes severer and more unapproachable with the increasing 

 height of the grass, the rapidity with which it advances is, by way 

 of compensation, proportionately diminished and the chances of 

 circumscribing it are in the same measure increased. From what 

 has just been said it must not be inferred that, because in short 

 grass the fire does not flare up high, it cannot therefore damage the 

 forest growth beyond a moderate distance above the ground. With 

 the burning grass barely more than a foot high, the heat vertically 

 over it may be intense enough to set ablaze the entire crowns of 

 trees 80 feet and upwards in height. Another very imporant point 

 to draw the attention of the student to is the rapidity with which fire 

 spreads when the grass is short without being quite spare. Before 

 an ordinary breeze, fire in such grass may sweep on at the rate of 

 3 to 4 miles an hour, and driven by a steady wind, still very far 

 short of a gale, it may rush forward at double that speed. 



(2) Dryness of the grass. It is evident that the more mois- 

 ture there is in the grass, the greater will be the difficulty with 

 which it will take fire, the less rapidly and less regularly will it 



