MANAGEMENT OF FIRE PROTECTION LINES. 203 



act as efficient spark catchers. All forest tires begin as sui-face 

 fires, and when the crowns of the trees are close to the ground, 

 the flames spread to them without difficulty. But when the crowns 

 are high, and beyond the reach of the flames, they remain 

 practically uninjured, and it then only becomes a question as to 

 how much heat the stems are able to bear. The power of 

 resisting heat is acquired by the Scots fir very early ; stems 

 of an inch or two in diameter acquiring a bark thick enough 

 to char outwardly without damaging the vitality of the tree, 

 while birch of the same size is much more sensitive, and oak, 

 beech, hornbeam, etc., on account of their thin periderm, are 

 quickly destroyed. A Scots fir plantation, therefore, can be made 

 secure against fire provided that precautions are taken to prevent 

 the fire from reaching the crown, and that the heat is not sufficient 

 to injure the stem. The height to which the flames will rise, and 

 the heat of the fire, depend upon — (1) the quantity and condition 

 of the combustible material on the surface, (2) the density of 

 the crop, and (3) the area over which the fire extends. These 

 three factors can be regulated to some extent by the forester ; but 

 the violence of the wind and the dryness of the air are conditions 

 over which he has no control, though they have a considerable 

 influence upon the fierceness of a fire. As a general rule, the 

 drier gi'ounds, on which fires most frequently occur, have only 

 a scanty surface covering of moss, lichens, stunted grass, and herbs, 

 between which the dry needles of firs or leaves of hardwoods 

 lie. To prevent surface tires with this kind of covering, it is 

 sufficient if the trees are pruned up for the first 3 feet 

 from the surface, and the fallen branches or other combustible 

 material raked up, provided that the trees are not too dense, nor 

 the area too large. But with heather or thick turf, the trees 

 must be pruned for at least double this height. The thick bark 

 of young Scots fir proves sufficient protection only when every 

 tree stands free, otherwise the flames will rise between two 

 stems to a dangerous height. Care must be taken that each 

 ti'ee stands singly, or at least a yard from its neighbour, 

 but, at the same time, too wide distances must be avoided, as 

 these would allow sparks to fly through the belt. ■ Scots fir of 

 fifteen to twenty years of age is generally safe against surface fires 

 which have not embraced wide areas, and they are, at the same 

 time, able to hinder sparks from reaching ground behind them. 

 With surface fires covering a wide area, the heated air enables 



