As wooden hulls were made larger they displayed a tendency, especially when they were built to carry propelling engines, to sag or hog, that is to say, to droop amidships or at the ends. This difficulty was ingeniously overcome in America, where wooden steamers were built longer and lighter and shallower than in Great Britain to suit the vast rivers of that country, by Stevens, who introduced his hogging frame, to which fuller reference has been made in Chapter II. But in the steamers of Great Britain, which were entirely for deep sea, this arrangement was impossible, and the solution of the difficulty had to be found in the use of a material other than wood. The only substitute was iron. The change from wood to iron meant a saving in weight of hull of about thirty to forty per cent., while it is asserted that in a few cases there has been an even greater difference. The saving also meant that the difference in weight could be added to the weight of the cargo, without increasing the displacement; while another advantage was that the beams and ribs and stringers were of smaller dimensions, and the space thus gained, added to that obtained by the substitution of thin iron plates for wooden planking several inches thick, also very considerably increased the space available for the stowage of cargo. Practically every part of a ship was of wood until 1810, in which year the scarcity of oak resulting from the extensive felling of trees in the English forests compelled the use of iron for the knees or connections between the deck-houses and the ribs, and for the breast-hooks and pillars of ships. An experimental iron barge was made in 1787 by J. Wilkinson the ironmaster. As early as 1809 it was proposed by Richard Trevithick and Robert Dickenson that ships should be built of iron, but the proposal was received with derision. The _Vulcan_, built in 1818 at Faskine near Glasgow, is, so far as is known, the first iron vessel constructed for commercial purposes, and so well was she built that as recently as 1875 she was engaged in transporting coal on the Forth and Clyde Canal, and looked little the worse for wear. Her builder was one Thomas Wilson.