There was something mysterious in this manoeuvre at first, but the secret of it was not kept for long.  An acrid smell stole out into the air, which thickened every minute in intensity.  Kettle seemed dimly to recognize it, but could not put a name to it definitely.  Besides, he was working with all his might at scraping away the earth from the foot of the wall, and had little leisure to think of other things.

The heat was stifling, and the sweat dripped from him, but he toiled on with a savage glee at his success.  The foundations had not been dug out; they were “floating” upon the earth surface; and the labor of undermining would, it appeared, be small.

But Murray in the other prison had smelt the reek before, and was able to put a name to it promptly.  “By Jove!  Captain,” he shouted mistily from the distance, “they’re going to smoke us to death; that’s the game.”

“Looks like trying it,” panted the little sailor, from his work.

“That’s dried camel’s dung they’re burning.  There’s no wood in Arabia here, and that’s their only fuel.  When the smoke gets into your lungs, it just tears you all to bits.  I say, Skipper, can’t you come to some agreement with Rad over those blessed rifles?  It’s a beastly death to die, this.”

“You aren’t dead—­by a long chalk—­yet.  More’m I. I’d hate to be—­smoke-dried like a ham—­as bad as any Jew.  But I don’t start in—­to scoff the cargo—­on my own ship—­at any bally price.”

There was a sound of distant coughing, and then the misty question:  “What are you working at?”

“Taking—­exercise,” Kettle gasped, and after that, communication between the two was limited to incessant staccato coughs.

More and more acrid grew the air as the burning camel’s dung saturated it further and further with smoke, and more and more frenzied grew Kettle’s efforts.  Once he got up and stuffed his coat in the embrasure from which the smoke principally came.  But that did little enough good.  The wall was all chinks, and the bitter reek came in unchecked.  He felt that the hacking coughs were gnawing away his strength, and just now the utmost output of his thews was needed.

He had given up his original idea of mining a passage under the wall.  Indeed, this would have been a labor of weeks with the poor broken crock which was his only tool, for the weight of the building above had turned the earth to something very near akin to the hardness of stone.  But he had managed to scrape out a space underneath one brick, and found that it was loosened, and with trouble could be dislodged; and so he was burrowing away the earth from beneath others, to drop more bricks down from their places, and so make a gangway through the solid wall itself.

But simple though this may be in theory, it was tediously difficult work in practice.  The bricks jammed even when they were undermined, and the wall was four bricks thick to its further side.  Moreover, every alternate course was cross-pinned, and the workman was rapidly becoming asphyxiated by the terrible reek which came billowing in from the chamber beyond.