They entered the burned area, and climbed on up the hill.  Though evidently here the ferocity of the conflagration had passed, it had left its rear guard behind.  Fallen trees still blazed; standing trees flamed like torches—­but all harmlessly within the magic circle drawn by the desperate quick work of the rangers.  They threaded their way cautiously among these isolated fires, watching lest some dead giant should fall across their path.  The ground smoked under their feet.  Against the background of a faint and distant roaring, which now made itself evident, the immediate surroundings seemed very quiet.  The individual cracklings of flames were an undertone.  Only once in a while a dull heavy crash smote the air as some great tree gave up the unequal struggle.

They passed as rapidly as they could through this stricken field.  The night had fallen, but the forest was still bright, the trail still plain.  They followed it for an hour until it had topped the lower ridge.

Then far ahead, down through the dark trunks of trees, they saw, wavering, flickering, leaping and dying, a line of fire.  In some places it was a dozen feet high; in others it sank to within a few inches of the ground—­but nowhere could the eye discern an opening through it.  A roar and a crackling filled the air.  Sparks were shooting upward in the suction.  A blast of heat rushed against Bob’s cheek.  All at once he realized that a forest fire was not a widespread general conflagration, like the burning of a city block.  It was a line of battle, a ring of flame advancing steadily.  All they had passed had been negligible.  Here was the true enemy, now charging rapidly through the dry, inflammable low growth, now creeping stealthily in the needles and among the rocks; always making way, always gathering itself for one of its wild leaps which should lay an entire new province under its ravaging.  Somewhere on the other side of that ring of fire were four men.  They were trying to cut a lane over which the fire could not leap.

Bob gazed at the wall of flame with some dismay.

“How we going to get through?” he asked.

“We got to find a rock outcrop somewheres up the ridge,” explained Jack, “where there’ll be a break in the fire.”

He turned up the side of the mountain again, leading the way.  After a time they came to an outcrop of the sort described, which, with some difficulty and stumbling, they succeeded in crossing.

Ahead, in the darkness, showed a tiny licking little fire, only a few inches high.

“The fire has jumped!” cried Bob.

“No, that’s their backfire,” Pollock corrected him.

They found this to be true.  The rangers had hastily hoed and raked out a narrow path.  Over this a very small fire could not pass; but there could be no doubt that the larger conflagration would take the slight obstacle in its stride.  Therefore the rangers had themselves ignited the small fire.  This would eat away the fuel, and automatically widen the path.  Between the main fire and the back fire were still several hundred yards of good, unburned country.  To Bob’s expression of surprise Amy added to the two principles of fire-fighting he had learned from Pollock.