The period which follows after about the year 1862 is notable as witnessing not only the gradual universal adoption of the screw in steamships, but the more general appreciation of iron as the material from which to construct a vessel’s hull. After the prejudices which already we have seen arising at different stages of the steamship’s history, it was scarcely to be wondered at that iron should come in for its full share of virulent criticism and opposition. The obvious remark made on all sides was that to expect iron to float was to suppose that man could act exactly contrary to the laws of Nature, and this notwithstanding that already, besides barges, a few ships thus built had somehow not only managed to keep afloat, but to traverse channels and oceans in perfect safety, carrying such heavy weights as their own machinery, to say nothing of their cargoes and human freights. But slowly the public prejudice began to wane. Already the Cunard Company had given way to iron in 1856, and in 1860 the Admiralty were at last convinced that the new method was just and sound. Within the limited scope at our command we have not space here to enter into the elaborate discussion of matters which have to be taken for granted before the building of the steamship begins. But the plain answer to the natural inquiry, as to how and why a vessel made out of iron does not immediately sink to the bottom as soon as ever she is launched, is this: whereas iron in itself is far heavier than water, yet the iron ship has not the same specific gravity as the iron from which it is made. Therefore, the ship of this material will be supported by the water in which it is placed. In actual displacement, an iron ship is proportionately lighter than a ship built of wood, and by “displacement” is meant the amount of water which a vessel displaces through being allowed to float. Of course, the quantity of water which a ship displaces (or pushes to one side) depends entirely on the weight of the vessel, and is exactly equal to the weight of the ship. Thus, suppose we were to fill a dock with water up to the level of the quay and then lower down into it by means of gigantic cranes a _Mauretania_ or _Lusitania_, the water would, of course, flow over on to the quays. Now the amount of water thus driven out would be the exact equivalent of the liner’s displacement. When we say, for instance, that the displacement of the _Mauretania_ is 40,000 tons, when loaded, we mean that her total weight when loaded is this number of tons, and her hull when afloat puts on one side (or “displaces”) just that amount of water.