disadvantage of having their speed diminished, after a short period of service, by the adhesion and growth of animal and vegetable matter. A coating of red lead is not a perfect preventive against this mischief, and various other scientific substitutes have been used of late; so that it cannot be doubted but this inconvenience will disappear altogether before scientific appliances. A perfectly protective varnish for the in sides of iron ships and a coating which shall effectually prevent the adhesion of animal and vegetable substances to the exterior, are desiderata of great value,[120] and will, we may hope, continue to receive the careful consideration of scientific men. [Sidenote: Durability, strength, and safety of iron.] All the facts yet known with regard to the superior _durability_[121] of iron ships are highly satisfactory. It is a consideration not to be overlooked that large ships may be rendered more durable than small vessels; for, as the weight of the hull is generally determined in a certain proportion to the whole displacement, and the plates of iron are much thicker in a large than in a small ship (the oxidizing causing an uniform waste of metal), the durability will be in proportion to the amount of wear the plates of the respective vessels can bear without danger to the ship. But the superior _strength_ of iron ships depends not merely upon the quality of the material employed, but also on the mode of combining it. The strength of wrought iron is well known and its power of resisting strains in almost every direction is a matter of universal experience, add to which, that its resistance to lateral pressure increases in a much higher ratio than the quantity of material. Hence, almost any amount of strength may be given to a large fabric; certainly, enough to bear the pressures and strains to which ships are exposed, with much less liability to injury than wood. With plates of iron of a substance fitly proportioned to the magnitude of the fabric, and with joints properly formed, the sides of ships have been found capable of resisting, in a remarkable manner, forces for which the strength of timber would be quite insufficient. A substance of plates sufficient to constitute this amount of strength generally, is also able to bear concussions of great force with much less hazard than timber. The uninjured state in which the _Great Britain_ was found in Dundrum Bay, after being wrecked and lying on the beach several months during winter, exposed to various storms, proved the correctness of these views, which more extended experience has since confirmed. Experience has also demonstrated that unless the concussion takes place with extreme violence, mere indentation of the metal is generally the greatest injury sustained. Beyond this, the strain sometimes breaks off the heads of a few rivets without opening the seam, but it is uncommon for the rivets to be drawn if the metal and workmanship are good. In