Climbing upon a high rock, the forester searched for the point at which the fire had originated. Prom his pocket he drew some powerful field-glasses, and again and again swept his vision over the farther edge of the burned area. Presently he closed his glasses and leaped to the ground. "Come on," he said, and headed diagonally across the burned tract. In a few minutes the three stood on the unburned forest floor on the farther side of the strip of black. "We must get our aerial up at once, Lew," said Charley. "It's been three-fourths of an hour since we talked to Willie." They glanced about, selected two suitable trees, and had the supporting wires attached to them in no time, with the aerial dangling aloft between the trees. It took only a moment more to couple up the instruments. "CBWC--CBWC--CBWC--CBC," rapped out Charley, as soon as the outfit was in readiness. Almost instantly Willie replied to the signal. "Any message for us from Oakdale?" inquired Charley. "Not a word. What are you doing?" "We are investigating the cause of the fire. Have moved our aerial down past the burned area. Forester and Lew and I alone. Fire crew on way back to Oakdale." "Have you found cause of fire?" "No. Just got here. Haven't investigated yet. Will listen in every quarter hour, beginning with the hour." "All right. I'll be here. Good-bye." The minute Charley finished talking with Willie, the three investigators set about their work. "We'll walk along the edge of the burned area," said the forester, "and try to find the point of origin." He went ahead, the two boys following. They were facing toward the brook. The line was irregular, like a huge saw-blade, with little jutting, black teeth here and there, where the flames had crept out in advance of the main line. The wind that had come up when the boys were fighting the fire had driven the flames back upon the area they had already consumed and the blaze had died out of itself. It could not eat its way to windward out here in the open, as it could have done in the dense timber where the wind was broken. From their starting-point they walked to the brook, finding nothing to enlighten them. They then retraced their steps, walking along the windward edge of the fire. Yet they found nothing to show them how or where the fire originated. "Evidently the flames have eaten their way some distance to windward of the point of origin," said the forester. "We shall have to look within the burned area." As he started to cross the black strip, the forester continued: "Perhaps I had better go through the burned strip alone. I want things disturbed as little as possible, and three will stir up the ashes a good deal more than one. You keep looking along the edge, and I'll search among the ashes." "Is there anything in particular we are to look for?" asked Charley. "Is there any special way to distinguish the starting-point of the fire?"