"Is it safe? I mean, are there any chasms?"
"Smooth as a floor. Keep close by me."
They examined the cavern carefully by means of the strange lantern filled with fungus, and Mr. Hume halted by the second exit.
"This is where they enter," he said, "and I think our best plan will be to build a fire in the mouth. We should then have the advantage over them, as we should see them once they came into the reflection."
They set about collecting wood, when Venning had a thought.
"Which way does the draught set in the tunnel—away from the cavern or into it?"
"Why?"
"Because, if the current of air blows away from us, we can easily keep them out."
"It blows from the cave into the tunnel. I found that out before."
"Then we have got them, whoever they are. Make the fire in the passage, pile up blocks of this sulphur on the inner side, and the wind will carry the fumes down into the tunnel."