And it is indeed an impressive sight to stand below these great
monsters of steel and watch them faithfully and untiringly pounding at
their work, all in order, and exactly in agreement with each other,
taking no notice of night or day, of storm or calm, but slinging along
at all times, doing their duty with an energetic goodwill which makes
them seem almost human--almost like gigantic Boy Scouts!
The great steel shaft which the four pistons keep driving round is
nearly 100 yards in length, and carries the big bronze screw propeller
at its end, which thrusts the ship along. There are two of these, one
on each side of the ship, which is therefore called a twin-screw
vessel.
There are four cylinders to each shaft, and the same lot of steam is
used, passing from one cylinder to the other, beginning with the small
high-pressure cylinder, which it enters at its highest strength,
something like 250 lb. to the square inch, and ending with the big
low-pressure cylinder, where the pressure is only about 5 lb.
Then there are numbers of other engines. One for condensing the salt
water from the sea and making it into fresh water for the boilers.
This is done by boiling up the salt water so that the watery part of
it becomes steam, while the salty part remains behind as salt; the
steam, when cooled, becomes fresh water, and is then fit to be used in
the boilers to make steam.
* * * * *