Even Shylock was on board, though no one knew it, for in the dead of night he had stolen quietly up the gang-plank and had hidden himself in an empty water-cask in the forecastle.

[Illustration:  “IN THE DEAD OF NIGHT SHYLOCK HAD STOLEN UP THE GANG-PLANK”]

“’Tisn’t Venice,” he said, as he sat down and breathed heavily through the bung of the barrel, “but it’s musty and damp enough, and, considering the cost, I can’t complain.  You can’t get something for nothing, even in Hades.”

VIII

ON BOARD THE “GEHENNA”

When the Gehenna had passed down the Styx and out through the beautiful Cimmerian Harbor into the broad waters of the ocean, and everything was comparatively safe for a while at least, Sherlock Holmes came down from the bridge, where he had taken his place as the commander of the expedition at the moment of departure.  His brow was furrowed with anxiety, and through his massive forehead his brain could be seen to be throbbing violently, and the corrugations of his gray matter were not pleasant to witness as he tried vainly to squeeze an idea out of them.

“What is the matter?” asked Demosthenes, anxiously.  “We are not in any danger, are we?”

“No,” replied Holmes.  “But I am somewhat puzzled at the bubbles on the surface of the ocean, and the ripples which we passed over an hour or two ago, barely perceptible through the most powerful microscope, indicate to my mind that for some reason at present unknown to me the House-boat has changed her course.  Take that bubble floating by.  It is the last expiring bit of aerial agitation of the House-boat’s wake.  Observe whence it comes.  Not from the Azores quarter, but as if instead of steering a straight course thither the House-boat had taken a sharp turn to the northeast, and was making for Havre; or, in other words, Paris instead of London seems to have become their destination.”

Demosthenes looked at Holmes with blank amazement, and, to keep from stammering out the exclamation of wonder that rose to his lips, he opened his bonbonniere and swallowed a pebble.

“You don’t happen to have a cocaine tablet in your box, do you?” queried Holmes.

“No,” returned the Greek.  “Cocaine makes me flighty and nervous, but these pebbles sort of ballast me and hold me down.  How on earth do you know that that bubble comes from the wake of the House-boat?”

“By my chemical knowledge, merely,” replied Holmes.  “A merely worldly vessel leaves a phosphorescent bubble in its wake.  That one we have just discovered is not so, but sulphurescent, if I may coin a word which it seems to me the English language is very much in need of.  It proves, then, that the bubble is a portion of the wake of a Stygian craft, and the only Stygian craft that has cleared the Cimmerian Harbor for years is the House-boat—­Q.E.D.”

“We can go back until we find the ripple again, and follow that, I presume,” sneered Le Coq, who did not take much stock in the theories of his great rival, largely because he was a detective by intuition rather than by study of the science.