-
cylinder compound engines, though made throughout of aethereum, and
consequently presenting an exceedingly handsome appearance, suggested
more the idea of an exquisite model in silver than anything else, the
pair occupying very little more space than those of one of the larger
Thames river steamers. The impression of diminutiveness and inadequacy
of power passed away, however, when the professor informed his
companions that the vapour would enter the high-pressure cylinder at the
astounding pressure of five thousand pounds to the square inch, and
that, though the engines themselves would only make fifty revolutions
per minute, the propeller, would be made, by means of speed-multiplying
gear, to revolve at the rate of one thousand times per minute in air of
ordinary atmospheric pressure.

"But how on earth do you manage to get your vapour up to that tremendous
pressure?" asked the colonel.

"Oh!" answered the professor, "that is a mere matter of mixing.
According to the proportions in which the crystals and the acid are
mingled together, so is the pressure of the vapour."

"And how do you mingle them together?" asked the lieutenant.

"This," said the professor, leading them up to a small boiler-like
vessel, "is the generator. The crystals are placed in a hopper at one
end, and the acid in that small tank at the other, from whence they are
respectively conducted along tubes into a small well in the bottom of
the generator, where, their proportions being regulated by the size of
the tubes through which they pass, they mingle and generate a vapour
having a pressure of five thousand pounds on the square inch. See,
there is the gauge, and it is now registering a pressure of five
thousand pounds."

"Good Heavens, man!" exclaimed the baronet, starting back; "you don't
mean to say that your generator is _now_, at this moment, subjected to
that enormous pressure of more than two tons per square inch? Supposing
it exploded, what would become of us?"

"We should be consumed in an instant by the fierce heat of the liberated
vapour," replied the professor calmly. "But," he continued, "you need
have no apprehension of an explosion. When that generator was being
made I had a second one constructed at the same time, precisely similar
in every respect, and this second one I tested to destruction, with the
satisfactory result that it endured without distress a pressure of
thirty-five tons per square inch, showed the first s