In 1831, there were a considerable number of paddle steamers running
along some of the rivers in England, and across the Channel to the
Continent. But there were no ocean steamers, properly so-called, and
there were no steamers used for warlike purposes. As in the case of
the wagon boilers, the boilers of the paddle steamers of 1831 were
most unsuited for resisting pressure. They were mere tanks, and there
was as much pressure when there was no steam in the boiler from the
weight of the water on the bottom, as there was at the top of the
boiler from the steam pressure when the steam was up. Under these
circumstances, again, from 3½ lb. to 5 lb. was all the pressure the
boilers were competent to bear, and as the engines ran at a slow
speed, they developed but a small amount of horse-power in relation to
their size. Moreover, as in the land engine, the connection between
the parts of the marine engine was such as to be incompetent to stand
the strain that would come upon it if a higher pressure, with a
considerable expansion, were used, and thus the consumption of coal
was very heavy; and we know that, having regard to the then
consumption, it was said, on high authority, it would be impossible
for a steamboat to traverse the Atlantic, as it could not carry fuel
enough to take it across; and indeed it was not until 1838 that the
Sirius and the Great Western did make the passage. The passage had
been made before, but it was not until 1838 that the passenger service
can be said to have commenced. In 1831, the marine boiler was supplied
with salt water, the hulls were invariably of wood, and the speed was
probably from eight to nine knots an hour. In 1881, the vessels are as
invariably either of iron or of steel, and I believe it will not be
very long before the iron disappears, giving place entirely to the
last mentioned metal. With respect to the term "steel," I am ready to
agree that it is impossible to say where, chemically speaking, iron
ends and steel begins. But (leaving out malleable cast iron) I apply
this term "steel" to any malleable ductile metal of which iron forms
the principal element and which has been in fusion, and I do so in
contradistinction to the metal which may be similar chemically, but
which has been prepared by the puddling process. Applying the term
steel in that sense, I believe, as I have said, it will not be very
long before plate-iron produced by the puddling process will cease to
be used for the purpose of building vessels. With respect to marine
engines, they are now supplied with steam from multiple tubed boilers,
the shells of which are commonly cylindrical. They are of enormous
strength, and made with every possible care, and carry from 80 lb. to
100 lb. pressure on the square inch.
It has been found, on the whole, more convenient to expand the steam
in two or more cylinders, rather than in one. I quite agree that, as a
mere matter of engineering science, there is no reason why the
expansion should not take place in a single cylinder, unless it be
that a single cylinder is cooled down to an extent which cannot be
overcome by jacketing, and which, therefore, destroys a portion of the
steam on its entering into the cylinder.
As regards the propeller, as we know, except in certain cases, the
paddle-wheel has practically disappeared, and the screw propeller is
all but universally employed. The substitution of the screw propeller
for the paddle enables the engine to work at a much higher number of
revolutions per minute, and thus a very great piston speed, some 600
ft. to 800 ft. per minute, is attained; and this, coupled with the
fairly high mean pressure which prevails, enables a large power to be
got from a comparatively small-sized engine. Speeds of 15 knots an
hour are now in many cases maintained, and on trial trips are not
uncommonly exceeded. Steam vessels are now the accepted vessels of
war. We have them in an armored state and in an unarmored state, but
when unarmored rendered so formidable, by the command which their
speed gives them of choosing their distance, as to make them, when
furnished with powerful guns, dangerous opponents even to the best
armored vessels.