aptain, tapping him on the
shoulder and making him take off his gaze for a moment from the sight,
"do you think you understand the engines by this time, eh?"

Bob only needed the hint to speak; and out he came with a whole volley
of questions.

"What is that thing there?" he asked, "the thing that goes round, I
mean."

"The paddle-shaft," replied the Captain; "it turns the wheels."

"And that other thing that goes up and down?"

"The piston-rod," said the old sailor. "It is this which turns the
shaft."

"Then, I want to know how the piston makes the shaft turn round, when it
only goes up and down itself?"

"The `eccentric' manages to do that, although it was a puzzle for a long
time to engineers to solve the problem--not until, I believe, Fulton
thought of this plan," said the Captain; and, he then went on to explain
how, in the old beam-engine of Watt, as well as in the earlier
contrivances for utilising steam-power, a fly-wheel was the means
adopted for changing the perpendicular action of the piston into a
circular motion. "Of course, though," he added, "this fly-wheel was
only available in stationary engines for pumping and so on; but, when
the principle of the eccentric was discovered later in the day, the
previously uneducated young giant, `Steam,' was then broken to harness,
so to speak, being thenceforth made serviceable for dragging railway-
carriages on our iron roads, and propelling ships without the aid of
sails, and against the wind even, if need be!"

"But what is steam?" was Bob's next query. "That's what I want to
know."

This fairly bothered the Captain.

"Steam?" he repeated, "steam, eh? humph! steam is, well let me see,
steam is--steam!"

Bob exploded at this, his merriment being shared by Nellie and Mrs
Gilmour, the latter not sorry for the old sailor's "putting his foot in
it" by a very similar blunder to that for which he had laughed at her
shortly before; while, as for Dick, the struggles he made to hide the
broad grin which would show on his face were quite comical and even
painful to witness.

The Captain pretended to get into a great rage; although his twinkling
eyes and suppressed chuckle testified that it was only pretence all the
time, though his passion was well simulated.

"I don't see anything to laugh at, you young rascal," he said to Bob.
"I'm sure I've given you quite as good a definition as you would find in
any of those `catechisms of common things'--catechisms o