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 31/2 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.