and the whole would form a solid body that, if
it struck a rock, might have a hole punched through
its outer skin, but scarcely through the inner skin,
which would dinge upwards. If the rock broke
through both, then the compartment system
would limit the damage to a small portion of the
vessel. With hot bitumen run in between the two
skins, no mischief could take place by rust. The
whole floor might, for greater security, be composed of three shins, making up a thickness of
six feet.
But this supposes the ordinary process of riveting the plates together. If the plates could all be welded together in one piece instead of riveted, the same strength would be obtained with two-thirds the thickness of metal, or with the same thickness of metal one-third more strength would be attained. Could this be done, a vessel might be constructed with the entire shell, deck and all, in a single piece of malleable iron, as entire as the skin of a whale.
But this has not yet been done. No; and there was once a time when no iron vessel at all had been built. One thing is certain — iron has been welded together in larger or smaller pieces, more or less perfectly, from a very distant date.
What, then, is welding?
Heating the surfaces of two or more pieces of iron to a pasty or just melting condition, and bringing them into close contact, free from all scale or dirt, in which case the two or more pieces become one.
Scale is produced by atmospheric air impinging upon hot iron, and no union can take place while scale exists between the surfaces. But without atmospheric contact scale does not form. If a polished watch-spring be bedded in powdered charcoal, covered over in a crucible, and kept red-hot for a week without atmospheric contact, and then be suffered to cool gradually, it will come out without loss of polish. Every one knows that in the burning of an ordinary tallow-candle carbon is formed in the shape of what is called the snuff, and that this snuff goes on increasing in bulk till it rises above the bladder of flame which encloses it. If the candle be considerably inclined out of the vertical position, the carbon is protruded beyond the wick, atmospheric contact is induced, and the carbon is burnt away.
If flame can be made to impinge on iron surfaces, so as to shut out the atmosphere, scale will not be formed; the flame consumes the oxygen of the atmosphere, and prevents it from approaching the iron.
In ordinary welding the iron is put into a furnace, or into a smith’s forge. It is rare that more or less scaling does not take place. The smith tries to prevent it by throwing sand on to the iron, which melts into glass, and so shuts out the atmosphere. In these cases the coal with sulphur and other impurities is brought into contact with the metal. What is wanted is, not the coal, but the hydrogen and carbon producing flame and heat by impact. It is therefore worth inquiring whether this cannot be done in a better mode and with better management. There does not seem to be much difficulty. The oxyhydrogen blowpipe gives us the most intense heat we know of. If therefore a reservoir of hydrogen gas under pressure be made to communicate with a pipe and nozzle, and atmospheric air under pressure be used in a similar mode, and fire be applied, where the two unite, the result is a welding heat. If instead of a single nozzle, a combination of nozzles, or pipes with a continuous row of holes, be used, a flame of any length may be produced. This flame might be made to impinge on the joints of iron planks the whole length of a ship’s side, in such a manner as not to burn away the sharp edges of the plates, but to cover the surface so as to prevent scale, and produce a welding heat through the whole length, and thus simple pressure would effect a welded joint. The metal would become homogeneous. If this can be done (and there seems no reason against it), we shall attain strength and durability hitherto unaccomplished.
The next question is that of propulsion — the power to propel, and also the instrument to propel with. Steam is at present our best power, but it has its disadvantages. The power is not generated at the moment of action. It needs a reservoir, and the reservoir may burst. Magnetism differs from this, if we could only use magnetism. Then steam needs fresh and pure water, not easily to be had on the ocean. Again: steam needs fuel, which is very bulky and ties up space, and is moreover explosive, under certain circumstances, as well as the steam it helps to create; but withal steam is the best power we know of at present, and we must work with it till we get a better. We call the engine a steam-engine, but it is in reality a heat-engine; the water gives out power in proportion to the heat it absorbs, and all power appears to resolve itself ultimately into heat. Whether it be the power of steam, or magnetism, or electricity, or wind or water, or the power of animal muscles — whether elastic power or the power of gravity — all seems to resolve itself into the question of heat.
Many disputes have arisen as to whether circular movement or rectilinear movement is best for the prime mover; but thus far opinion seems to be in favour of the latter; and the next question is as to the instrument. The paddle, the screw, and the pump are up to this time the efficient means. Steam-moved oars have been tried, but with inefficient results. The paddle is out of the question for war-ships, even supposing it more efficient for speed than the screw. The beet made paddles should enter the water at one corner, but so as to make no blow, and in that case they would approximate to the action of the screw; but the side of the vessel would be a very awkward position for the screw. It is proved that the beet screws are those that enter the water gradually, with rounded corners.
The first propulsion by pump was tried by Dr. Franklin, who got astride of a wooden ship’s- pump in a pond, and found that by working the handle he could move himself along. The last trial was by a centrifugal pump, similar to that shown at the Great Exhibition, pulling the water in at the fore end of the vessel and discharging it on each side abaft. A single pump was used, and it was found that by delivering the water at a