All these steamers were built of wood, and as they increased in size they developed a marked tendency to “sag,” that is, drop in the middle, or to “hog,” that is, drop at the ends. This tendency was overcome by an ingenious system of stump-masts and strutts, and iron ties, invented by Colonel Stevens. There are various methods of applying these stiffeners, and the peculiar framework of wooden arches and stump-masts which appears on so many American river steamers is due to the necessity of employing one or other of these systems for strengthening purposes. In some of the later vessels (as in the _De Witt Clinton_) these ties are put into the framework of the superstructure. In construction, the development of American steamers on inland waters since Fulton’s time has proceeded on entirely different lines from those which marked the progress of river navigation in Great Britain. American river steamers were designed not only to cope with the traffic in narrower and shallower places, but to carry whatever was necessary in deeper waters, and at the same time get through the more difficult places somehow. The great distances to be travelled on the American rivers rendered necessary the provision of vessels carrying large quantities of cargo and extensive accommodation for passengers, whilst the bars occurring at intervals in the beds of the rivers made it compulsory that the vessels should be of light draught. The construction of English river steamers, on the other hand, has been conditioned by the comparative narrowness of the English rivers and the lowness of the many bridges which span them. [Illustration: THE “DE WITT CLINTON.” BUILT 1828.]