The year 1840, then, may be considered as one of the vital years in the progress of steam navigation. Since that time no year has passed without seeing some important addition and improvement made in the conquest of the ocean, either in size, shape, or speed of the "greyhounds." SHIPS BUILT OF IRON AND STEEL Even before the introduction of steam as a motive power for boats shipbuilders had been casting about for some satisfactory substitute for wood in the construction of vessels. One reason for this was that suitable wood was becoming scarce and very expensive. But also there was a limit to the size that a wooden vessel might be built with safety. A wooden boat more than three hundred feet long cannot be constructed without having dangerous structural weakness. Naturally the idea that the only suitable material for boat-building was something lighter than water,--something that would float--which had been handed down traditionally for thousands of years, could not be overcome in a moment. And surely such a heavy substance as iron would not be likely to suggest itself to the average ship-builder. But at the beginning of the nineteenth century rapid strides were being made in theoretical, as well as applied science, and the idea of using metal in place of wood for shipbuilding began to take practical form. Richard Trevithick, whose remarkable experiments in locomotive building have been noted in another chapter, had planned an iron ship as early as 1809. He did not actually construct a vessel, but he made detailed plans of one--not merely a boat with an iron hull, but with decks, beams, masts, yards, and spars made of the same material. It was nearly ten years after Trevithick drew his plans, however, before the first iron ship was constructed. Then Thomas Wilson of Glasgow built a vessel on practically the same lines suggested by Trevithick. This vessel, finished in 1818, and called the _Vulcan_, was the pioneer of all iron boats. For at least sixty years it remained in active service. Indeed, for aught that is known to the contrary, this first iron boat may be still in use in some capacity. One of the most surprising and interesting things to shipbuilders about the _Vulcan_, and the boats that were constructed after her, was the fact that they were actually lighter in proportion to their carrying capacity than ships of corresponding size built of wood. In wooden cargo ships the weight of the hull and fittings varies from 35 to 45 per cent. of the total displacement, while iron vessels vary from 25 to 30 per cent. This was a vital point in favor of the iron vessel, and one that appealed directly to practical builders. But the public at large looked askance at the new vessels. To "sink like a stone" was proverbial; and everyone knows that iron sinks quite as readily as stone.