"I wish we knew," replied his father, "then we might be prepared for it." But no preparation within their power could have availed against the cunning of the next assault. They had been watching and waiting half an hour or more in the darkness, when again the red shine of fire began to glow on the walls below them. "What is this?" muttered Mr. Haydon. "This light is far too strong for torches." And now with the gleam of fire came gusts of heat sweeping up to them, and clouds of thick pungent smoke. Half choked, and with smarting eyes, they watched for the fire to appear. Presently they saw it below them, and saw that a furnace of leaping flame was advancing towards them, flame which filled the whole of the space, licking walls, roof, and floor. They watched it with horrified eyes. It was impossible to meet this subtle and dreadful enemy with spear or _dah_. "What is it?" cried Jack. "A cunning trick, a cruelly cunning trick," replied his father. "They are thrusting great burning bundles of dried reeds and grass before them. The draught comes up the stairs and keeps the air cool and sweet for them, while it drives suffocating smoke and heat upon us." Jack ground his teeth as he saw how perfectly the plan was calculated to drive them out of the staircase into the open room above, where the numbers of the Kachins could be used to deadly purpose. "The fire is flagging," gasped Jack. "For the moment, yes," said his father. The glowing mass of flames wavered and began to sink. Then they saw how it was fed. A huge bundle of dried canes and reeds on the end of a spear was thrust into the flickering glow, and at once took fire and burned with the utmost fury. Fresh bundles were pushed forward beside it, and these, too, flared up with a shrill crackle of snapping canes and the roar of a fire fanned by a strong draught. Inch by inch the flames moved forward, themselves a terrible enemy, and behind them crept up and up a savage and merciless foe. CHAPTER XLI. THE SECRET PASSAGE. Within that confined space, the heat became that of a fiery furnace, the pungent smoke became overpowering. "We must get back or we shall be overcome," gasped Mr. Haydon, and they climbed the steep steps of stone. "Who's here?" snapped Mr. Haydon, as they turned the last bend. Jack looked under his father's arm. "It's the woman," he gasped, for the pungent smoke had almost stopped his breath. "She's come back. Where has she been?" Now the woman's voice came to them calling earnestly, "Sahib, sahib, sahib!" she cried. Jack and his father leapt into the room, where the wider space, though dim with smoke, made the air taste wonderfully fresh and sweet after the choking passage. The woman at once sprang at Mr. Haydon and seized his arm, talking earnestly. As she spoke, the elder man's face lighted up with a great hope. "Jack! Jack!" he cried. "Come on! come on! Here's a wonderful chance turned up."