"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 north-east, 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